feat(svg): add SVG path parsing and rendering (#6)

Add support for drawing SVG path data onto PDF pages. This enables
rendering icons, logos, and vector graphics from SVG files.

- Parse all SVG path commands (M/L/H/V/C/S/Q/T/A/Z and relative)
- Convert arcs to bezier curves for PDF compatibility
- Add PathBuilder.appendSvgPath() and PDFPage.drawSvgPath()
- Auto-transform Y-axis so SVG coords render correctly in PDF
- Include comprehensive tests and three usage examples

Author: Mythie

.agents/plans/042-svg-path-support.md

@@ -0,0 +1,233 @@
+# SVG Path Support
+
+## Problem Statement
+
+Users want to draw SVG path data onto PDF pages. A common use case is sewing patterns and technical drawings stored as SVG that need to be rendered to PDF.
+
+**User request from Reddit:**
+
+> I am currently using both PDFKit and pdfjs to create printable sewing patterns from SVG data. I currently have to take all my SVG path data, put it into an A0 PDF, load that PDF into a canvas element, then chop up the canvas image data into US letter sizes.
+
+## Goals
+
+1. Parse SVG path `d` attribute strings and render them via `PathBuilder`
+2. Support all SVG path commands (M, L, H, V, C, S, Q, T, A, Z) in both absolute and relative forms
+3. Integrate cleanly with existing `PathBuilder` API
+4. Flip the Y-axis by default so raw SVG coordinates map correctly into PDF space (with an opt-out)
+
+## Non-Goals
+
+- Full SVG document parsing (elements like `<text>`, `<image>`, `<use>`, CSS, filters)
+- SVG transforms (users can apply PDF transforms separately)
+- SVG units conversion (assume unitless = points, like the rest of our API)
+- Page tiling/splitting (users handle this themselves with our primitives)
+
+## Scope
+
+**In scope:**
+
+- SVG path `d` string parser
+- All path commands: M, m, L, l, H, h, V, v, C, c, S, s, Q, q, T, t, A, a, Z, z
+- Arc-to-bezier conversion for the `A` command
+- `PathBuilder.appendSvgPath()` instance method
+- `PDFPage.drawSvgPath()` convenience method
+
+**Out of scope:**
+
+- Helper to extract paths from SVG documents (maybe later as a separate utility)
+- Viewbox/coordinate system transforms
+- Stroke/fill style parsing from SVG attributes
+
+---
+
+## Desired Usage
+
+### Basic: Draw SVG path data
+
+```typescript
+// Convenience method on PDFPage - fill by default
+page.drawSvgPath("M 10 10 L 100 10 L 100 100 Z", {
+  color: rgb(1, 0, 0),
+});
+
+// With stroke
+page.drawSvgPath("M 10 10 C 20 20, 40 20, 50 10", {
+  borderColor: rgb(0, 0, 0),
+  borderWidth: 2,
+});
+```
+
+### Using PathBuilder for more control
+
+```typescript
+// When you need to choose fill vs stroke explicitly
+page
+  .drawPath()
+  .appendSvgPath("M 10 10 L 100 10 L 100 100 Z")
+  .stroke({ borderColor: rgb(0, 0, 0) });
+```
+
+### Chaining with existing PathBuilder methods
+
+```typescript
+page
+  .drawPath()
+  .moveTo(0, 0)
+  .appendSvgPath("l 50 50 c 10 10 20 20 30 10") // relative commands continue from current point
+  .lineTo(200, 200)
+  .close()
+  .stroke();
+```
+
+### Complex paths (sewing patterns, icons)
+
+```typescript
+// Heart shape
+page
+  .drawPath()
+  .appendSvgPath("M 10,30 A 20,20 0,0,1 50,30 A 20,20 0,0,1 90,30 Q 90,60 50,90 Q 10,60 10,30 Z")
+  .fill({ color: rgb(1, 0, 0) });
+
+// Multiple subpaths
+page
+  .drawPath()
+  .appendSvgPath("M 0 0 L 100 0 L 100 100 L 0 100 Z M 25 25 L 75 25 L 75 75 L 25 75 Z")
+  .fill({ windingRule: "evenodd" }); // Creates a square with a square hole
+```
+
+---
+
+## Architecture
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│  PathBuilder.fromSvgPath() / .appendSvgPath()               │
+│  (Entry points - high-level API)                            │
+├─────────────────────────────────────────────────────────────┤
+│  src/svg/path-parser.ts                                     │
+│  (Parse d string → command objects)                         │
+├─────────────────────────────────────────────────────────────┤
+│  src/svg/path-executor.ts                                   │
+│  (Execute commands via callback interface)                  │
+│  - Handles relative → absolute conversion                   │
+│  - Handles smooth curve reflection                          │
+│  - Handles arc → bezier conversion                          │
+├─────────────────────────────────────────────────────────────┤
+│  PathBuilder (existing)                                     │
+│  (moveTo, lineTo, curveTo, quadraticCurveTo, close)         │
+└─────────────────────────────────────────────────────────────┘
+```
+
+The `src/svg/` module is intentionally decoupled from `PathBuilder`. The executor takes a callback interface, so it can drive any path-building target (PathBuilder, canvas, testing, etc.).
+
+### New Files
+
+| File                       | Purpose                                                                            |
+| -------------------------- | ---------------------------------------------------------------------------------- |
+| `src/svg/path-parser.ts`   | Tokenize and parse SVG path `d` strings into command objects                       |
+| `src/svg/path-executor.ts` | Execute parsed commands with state tracking (relative coords, smooth curves, arcs) |
+| `src/svg/arc-to-bezier.ts` | Arc endpoint → center parameterization and bezier approximation                    |
+| `src/svg/index.ts`         | Public exports                                                                     |
+
+### Modified Files
+
+| File                              | Changes                                 |
+| --------------------------------- | --------------------------------------- |
+| `src/api/drawing/path-builder.ts` | Add `appendSvgPath()` instance method   |
+| `src/api/pdf-page.ts`             | Add `drawSvgPath()` convenience method  |
+| `src/index.ts`                    | Export svg utilities for advanced users |
+
+---
+
+## SVG Path Command Reference
+
+| Command | Parameters                      | Description      | PathBuilder equivalent                        |
+| ------- | ------------------------------- | ---------------- | --------------------------------------------- |
+| M/m     | x y                             | Move to          | `moveTo(x, y)`                                |
+| L/l     | x y                             | Line to          | `lineTo(x, y)`                                |
+| H/h     | x                               | Horizontal line  | `lineTo(x, currentY)`                         |
+| V/v     | y                               | Vertical line    | `lineTo(currentX, y)`                         |
+| C/c     | x1 y1 x2 y2 x y                 | Cubic bezier     | `curveTo(...)`                                |
+| S/s     | x2 y2 x y                       | Smooth cubic     | Reflect last CP, then `curveTo(...)`          |
+| Q/q     | x1 y1 x y                       | Quadratic bezier | `quadraticCurveTo(...)`                       |
+| T/t     | x y                             | Smooth quadratic | Reflect last CP, then `quadraticCurveTo(...)` |
+| A/a     | rx ry angle large-arc sweep x y | Elliptical arc   | Convert to bezier curves                      |
+| Z/z     | (none)                          | Close path       | `close()`                                     |
+
+**Lowercase = relative coordinates** (offset from current point)
+**Uppercase = absolute coordinates**
+
+---
+
+## Test Plan
+
+### Unit Tests
+
+**Parser tests (`src/svg/path-parser.test.ts`):**
+
+- Basic commands: M, L, H, V, C, Q, Z
+- Relative commands: m, l, h, v, c, q, z
+- Smooth curves: S, s, T, t
+- Arcs: A, a (various flag combinations)
+- Number formats: integers, decimals, negative, scientific notation
+- Whitespace variations: spaces, commas, no separators
+- Repeated commands (implicit repetition)
+- Invalid input handling (malformed paths)
+
+**Executor tests (`src/svg/path-executor.test.ts`):**
+
+- Relative to absolute conversion
+- Smooth curve control point reflection
+- Arc to bezier conversion accuracy
+- State tracking across commands
+
+**Arc conversion tests (`src/svg/arc-to-bezier.test.ts`):**
+
+- Various arc flag combinations (large-arc, sweep)
+- Degenerate cases (zero radii, same start/end point)
+- Accuracy of bezier approximation
+
+### Integration Tests
+
+**PathBuilder integration:**
+
+- `fromSvgPath()` produces correct operators
+- `appendSvgPath()` continues from current point
+- Chaining with other PathBuilder methods
+- Complex real-world paths (icons, shapes)
+
+### Visual Tests
+
+- Generate PDFs with various SVG paths
+- Compare with SVG rendered in browser
+- Test paths from real-world sources (Font Awesome icons, map data)
+
+### Edge Cases
+
+- Empty path string
+- Path with only M command (no drawing)
+- Very large coordinates
+- Very small arc radii (degenerate to line)
+- Zero-length arcs
+- Arcs with rx=0 or ry=0 (should become lines per SVG spec)
+
+---
+
+## Open Questions
+
+1. **Error handling**: Should malformed paths throw or silently skip bad commands?
+
+- **Recommendation**: Skip bad commands with console warning, continue parsing. Matches browser behavior.
+
+2. **Coordinate precision**: Should we round coordinates?
+
+- **Recommendation**: No rounding, preserve full precision. PDF handles it fine.
+
+---
+
+## Future Enhancements (Not in this plan)
+
+- `parseSvgPaths(svgDocument: string)`: Extract `<path>` elements with basic styles
+- Transform parsing (`transform` attribute)
+- Style extraction (`fill`, `stroke`, `stroke-width` attributes)
+- Support for other SVG shape elements (`<rect>`, `<circle>`, `<ellipse>`, `<polygon>`)

CODE_STYLE.md

@@ -91,20 +91,66 @@ if (condition) return early;
 if (condition) {
   return early;
 }
+```
 
-// Bad: single-line else
-if (condition) {
-  doSomething();
-} else doOther();
+### Prefer Early Returns Over Else
 
-// Good: braces on else too
-if (condition) {
-  doSomething();
-} else {
-  doOther();
+Avoid `else` and `else if` when possible. Early returns reduce nesting and make code easier to follow — once you hit an `else`, you have to mentally track "what was the condition again?" which is annoying.
+
+```typescript
+// Bad: else creates unnecessary mental context-switching
+function getStatus(user: User): string {
+  if (user.isAdmin) {
+    return "admin";
+  } else if (user.isModerator) {
+    return "moderator";
+  } else {
+    return "user";
+  }
+}
+
+// Good: early returns, flat structure
+function getStatus(user: User): string {
+  if (user.isAdmin) {
+    return "admin";
+  }
+
+  if (user.isModerator) {
+    return "moderator";
+  }
+
+  return "user";
+}
+
+// Bad: nested else blocks
+function processData(data: Data | null): Result {
+  if (data) {
+    if (data.isValid) {
+      return compute(data);
+    } else {
+      throw new Error("Invalid data");
+    }
+  } else {
+    throw new Error("No data provided");
+  }
+}
+
+// Good: guard clauses with early returns
+function processData(data: Data | null): Result {
+  if (!data) {
+    throw new Error("No data provided");
+  }
+
+  if (!data.isValid) {
+    throw new Error("Invalid data");
+  }
+
+  return compute(data);
 }
 ```
 
+Sometimes `else` is unavoidable (e.g., ternaries, complex branching where both paths continue), but don't reach for it by default.
+
 ## Naming Conventions
 
 ### Classes