Add SVGElementType protocol and element conformances

Add SVGElementType protocol to define common requirements for all SVG
element types (tagName and isSelfClosing properties). Update all 20
W3C_SVG2 element types to conform to this protocol:

- Document elements: Group, SVG, Defs, Use, Symbol
- Shape elements: Circle, Ellipse, Line, Rectangle, Polygon, Polyline
- Text elements: Text, TSpan
- Painting elements: ClipPath, Mask, Marker
- PaintServer elements: LinearGradient, RadialGradient, Pattern, Stop

This enables type-safe SVG element rendering with protocol-based
polymorphism while maintaining the existing element structure.

All tests pass (79 tests in 29 suites).

Author: coenttb

Sources/W3C SVG/SVGElementType.swift

@@ -0,0 +1,23 @@
+//
+//  SVGElementType.swift
+//  swift-w3c-svg
+//
+//  Created by Coen ten Thije Boonkkamp
+//
+
+/// Protocol that all SVG element types must conform to.
+///
+/// This protocol defines the basic requirements for SVG elements,
+/// allowing for type-safe element construction with specific parameters.
+public protocol SVGElementType: Sendable {
+    /// The tag name of the SVG element (e.g., "rect", "circle", "path")
+    static var tagName: String { get }
+
+    /// Whether this element is self-closing (has no children)
+    static var isSelfClosing: Bool { get }
+}
+
+extension SVGElementType {
+    /// Default implementation - most SVG elements can have children
+    public static var isSelfClosing: Bool { false }
+}

Sources/W3C SVG/W3C_SVG2.Document.Defs.swift

@@ -40,7 +40,7 @@ extension W3C_SVG2.Document {
     /// - ``Symbol``
     /// - ``Use``
     /// - ``Group``
-    public struct Defs: Sendable, Equatable {
+    public struct Defs: SVGElementType, Sendable, Equatable {
         /// Optional identifier for the defs element
         public let id: String?
 

Sources/W3C SVG/W3C_SVG2.Document.Group.swift

@@ -39,7 +39,7 @@ extension W3C_SVG2.Document {
     ///
     /// - ``SVG``
     /// - ``Defs``
-    public struct Group: Sendable, Equatable {
+    public struct Group: SVGElementType, Sendable, Equatable {
         /// Optional identifier for the group
         ///
         /// Used for referencing, animation, or styling

Sources/W3C SVG/W3C_SVG2.Document.SVG.swift

@@ -40,7 +40,7 @@ extension W3C_SVG2.Document {
     ///
     /// - ``Group``
     /// - ``Symbol``
-    public struct SVG: Sendable, Equatable {
+    public struct SVG: SVGElementType, Sendable, Equatable {
         /// The x-axis coordinate for embedded svg elements
         ///
         /// Default value: 0

Sources/W3C SVG/W3C_SVG2.Document.Symbol.swift

@@ -54,7 +54,7 @@ extension W3C_SVG2.Document {
     /// - ``Use``
     /// - ``Defs``
     /// - ``SVG``
-    public struct Symbol: Sendable, Equatable {
+    public struct Symbol: SVGElementType, Sendable, Equatable {
         /// Identifier for referencing the symbol
         public let id: String?
 

Sources/W3C SVG/W3C_SVG2.Document.Use.swift

@@ -54,7 +54,7 @@ extension W3C_SVG2.Document {
     /// - ``Symbol``
     /// - ``Defs``
     /// - ``Group``
-    public struct Use: Sendable, Equatable {
+    public struct Use: SVGElementType, Sendable, Equatable {
         /// URL reference to target element
         ///
         /// Format: "#elementId" for same-document references

Sources/W3C SVG/W3C_SVG2.PaintServers.LinearGradient.swift

@@ -48,7 +48,7 @@ extension W3C_SVG2.PaintServers {
     ///
     /// - ``RadialGradient``
     /// - ``Stop``
-    public struct LinearGradient: Sendable, Equatable {
+    public struct LinearGradient: SVGElementType, Sendable, Equatable {
         /// Identifier for referencing the gradient
         public let id: String?
 

Sources/W3C SVG/W3C_SVG2.PaintServers.Pattern.swift

@@ -51,7 +51,7 @@ extension W3C_SVG2.PaintServers {
     ///
     /// - ``LinearGradient``
     /// - ``RadialGradient``
-    public struct Pattern: Sendable, Equatable {
+    public struct Pattern: SVGElementType, Sendable, Equatable {
         /// Identifier for referencing the pattern
         public let id: String?
 

Sources/W3C SVG/W3C_SVG2.PaintServers.RadialGradient.swift

@@ -52,7 +52,7 @@ extension W3C_SVG2.PaintServers {
     ///
     /// - ``LinearGradient``
     /// - ``Stop``
-    public struct RadialGradient: Sendable, Equatable {
+    public struct RadialGradient: SVGElementType, Sendable, Equatable {
         /// Identifier for referencing the gradient
         public let id: String?
 

Sources/W3C SVG/W3C_SVG2.PaintServers.Stop.swift

@@ -38,7 +38,7 @@ extension W3C_SVG2.PaintServers {
     ///
     /// - ``LinearGradient``
     /// - ``RadialGradient``
-    public struct Stop: Sendable, Equatable {
+    public struct Stop: SVGElementType, Sendable, Equatable {
         /// Position along gradient vector
         ///
         /// Format: number (0–1) or percentage (0%–100%)