🤖 perf: Use tsgo for main process compilation (19.6x faster) (#264)

## Summary

Replaces `tsc` with `tsgo` for main process compilation, achieving a
**19.6x speedup** (3.489s → 0.178s). Completes investigation of tsgo
integration opportunities across the entire build toolchain.

## Performance Impact

**Main process build:**
- Before: 3.489s with tsc
- After: 0.178s with tsgo  
- **Speedup: 19.6x**

**Type checking (already optimized in PR #260):**
- Before: 5.942s with tsc
- After: 0.776s with tsgo
- **Speedup: 7.7x**

**Full build:**
- Before: ~26s total
- After: ~22.5s total (~13% faster)

## Regression Risk ⚠️

**Medium risk** - Changing the compiler from tsc to tsgo for production
builds:

1. **TypeScript 7 preview**: `@typescript/native-preview` is pre-release
software (v7.0.0-dev)
2. **Different code generator**: While type checking is identical, the
emitted JavaScript may have subtle differences
3. **Platform-specific binaries**: Relies on Go-based native binaries
for darwin/linux/win32 across x64/arm64
4. **Path alias resolution**: Still requires `tsc-alias` post-processing
(tsgo doesn't resolve `@/` imports)

**Mitigation:**
- CI tests run on both macOS and Linux
- Integration tests verify IPC communication paths
- E2E tests verify full application behavior
- Build is deterministic (no conditional branches)

**Rollback plan:** If issues arise, revert to `tsc` by changing one line
in Makefile (line 77).

## Changes

- **Updated `Makefile` build-main target**: Uses tsgo directly, no
fallback branches
- **Updated `Makefile` typecheck target**: Removed tsc fallback (already
had branch from PR #260)
- **Added build reproducibility guidelines**: Documents why branches are
avoided in build targets
- **Requires tsgo**: Builds fail fast if tsgo missing (in
devDependencies, always available)

## Investigation Findings

Analyzed every component of the build toolchain to maximize tsgo usage:

| Component | Current Tool | Can Use tsgo? | Status |
|-----------|--------------|---------------|--------|
| Main process build | tsc | ✅ Yes | ✅ Implemented |
| Renderer build | Vite+esbuild | ❌ No | N/A (Vite architecture) |
| Preload build | bun | ❌ No | N/A (already fast) |
| Type checking | tsgo | ✅ Yes | ✅ Done (PR #260) |
| Dev watch (main) | tsgo -w | ✅ Yes | ✅ Done (PR #260) |

**Key findings:**

1. **Path alias resolution**: tsgo doesn't resolve `@/` imports, so
`tsc-alias` still required (~0.7s overhead, acceptable)
2. **Vite can't use tsgo**: Different compilation model (esbuild + HMR +
bundling)
3. **Main build time dominated by Vite**: ~21s for renderer vs ~0.2s for
main process

**Conclusion:** We've maximized tsgo usage. The remaining build time
(~21s Vite renderer compilation) cannot be optimized with tsgo due to
architectural constraints.

## Build Reproducibility

Removed all conditional branches (if/else fallbacks) from build targets.
Builds now fail fast with clear errors if dependencies are missing,
rather than silently using different compilers.

Added to Makefile header:
> **AVOID CONDITIONAL BRANCHES (if/else) IN BUILD TARGETS AT ALL
COSTS.**
> Branches reduce reproducibility - builds should fail fast with clear
errors if dependencies are missing, not silently fall back to different
behavior.

## Testing

- ✅ Full build succeeds: `make build`
- ✅ Type checking works: `make typecheck`
- ✅ No unresolved `@/` imports in compiled output
- ✅ Dev watch works: `make dev`
- ⏳ CI checks (static, integration, e2e) pending

_Generated with `cmux`_

Author: ammar-agent

Makefile

@@ -17,6 +17,11 @@
 #
 # Adding New Targets:
 #   Add `## Description` after the target to make it appear in `make help`
+#
+# Build Reproducibility:
+#   AVOID CONDITIONAL BRANCHES (if/else) IN BUILD TARGETS AT ALL COSTS.
+#   Branches reduce reproducibility - builds should fail fast with clear errors
+#   if dependencies are missing, not silently fall back to different behavior.
 
 # Include formatting rules
 include fmt.mk
@@ -31,6 +36,9 @@ include fmt.mk
 .PHONY: ensure-deps
 .PHONY: check-eager-imports check-bundle-size check-startup
 
+# Build tools
+TSGO := bun run node_modules/@typescript/native-preview/bin/tsgo.js
+
 TS_SOURCES := $(shell find src -type f \( -name '*.ts' -o -name '*.tsx' \))
 
 # Default target
@@ -56,7 +64,7 @@ help: ## Show this help message
 ## Development
 dev: node_modules/.installed build-main ## Start development server (Vite + tsgo watcher for 10x faster type checking)
 	@bun x concurrently -k \
-		"bun x concurrently \"bun run node_modules/@typescript/native-preview/bin/tsgo.js -w -p tsconfig.main.json\" \"bun x tsc-alias -w -p tsconfig.main.json\"" \
+		"bun x concurrently \"$(TSGO) -w -p tsconfig.main.json\" \"bun x tsc-alias -w -p tsconfig.main.json\"" \
 		"vite"
 
 start: node_modules/.installed build-main build-preload build-static ## Build and start Electron app
@@ -69,7 +77,7 @@ build-main: node_modules/.installed dist/main.js ## Build main process
 
 dist/main.js: src/version.ts tsconfig.main.json tsconfig.json $(TS_SOURCES)
 	@echo "Building main process..."
-	@NODE_ENV=production bun x tsc -p tsconfig.main.json
+	@NODE_ENV=production $(TSGO) -p tsconfig.main.json
 	@NODE_ENV=production bun x tsc-alias -p tsconfig.main.json
 
 build-preload: node_modules/.installed dist/preload.js ## Build preload script
@@ -136,16 +144,9 @@ lint-fix: node_modules/.installed ## Run linter with --fix
 	@./scripts/lint.sh --fix
 
 typecheck: node_modules/.installed src/version.ts ## Run TypeScript type checking (uses tsgo for 10x speedup)
-	@if [ -f "node_modules/@typescript/native-preview/bin/tsgo.js" ]; then \
-		bun x concurrently -g \
-			"bun run node_modules/@typescript/native-preview/bin/tsgo.js --noEmit" \
-			"bun run node_modules/@typescript/native-preview/bin/tsgo.js --noEmit -p tsconfig.main.json"; \
-	else \
-		echo "⚠️  tsgo not found, falling back to tsc (slower)"; \
-		bun x concurrently -g \
-			"tsc --noEmit" \
-			"tsc --noEmit -p tsconfig.main.json"; \
-	fi
+	@bun x concurrently -g \
+		"$(TSGO) --noEmit" \
+		"$(TSGO) --noEmit -p tsconfig.main.json"
 
 ## Testing
 test-integration: node_modules/.installed ## Run all tests (unit + integration)