fix: cache canvas 2D context and lock context type per spec

Per the HTML spec, HTMLCanvasElement.getContext() should:
1. Return the same object on repeated calls with the same type
2. Return null if a different context type was already requested

Previously, every getContext("2d") call created a new
CanvasRenderingContext2D object. This caused issues with code
that relies on identity checks (ctx === canvas.getContext("2d"))
and wasted memory by allocating duplicate contexts.

The fix caches the 2D context and tracks which context type was
first requested, returning null for incompatible subsequent calls.

src/browser/webapi/element/html/Canvas.zig

@@ -30,6 +30,13 @@ const OffscreenCanvas = @import("../../canvas/OffscreenCanvas.zig");
 const Canvas = @This();
 _proto: *HtmlElement,
 
+/// Cached context type. Once set, requesting a different type returns null (per spec).
+_context_type: ContextType = .none,
+/// Cached 2D rendering context (same object returned on repeated getContext("2d") calls).
+_ctx_2d: ?*CanvasRenderingContext2D = null,
+
+const ContextType = enum { none, @"2d", webgl };
+
 pub fn asElement(self: *Canvas) *Element {
     return self._proto._proto;
 }
@@ -69,12 +76,23 @@ const DrawingContext = union(enum) {
 
 pub fn getContext(self: *Canvas, context_type: []const u8, page: *Page) !?DrawingContext {
     if (std.mem.eql(u8, context_type, "2d")) {
+        // Return cached context if available.
+        if (self._ctx_2d) |cached| return .{ .@"2d" = cached };
+        // Per spec: return null if a different context type was already requested.
+        if (self._context_type != .none) return null;
+
         const ctx = try page._factory.create(CanvasRenderingContext2D{ ._canvas = self });
+        self._ctx_2d = ctx;
+        self._context_type = .@"2d";
         return .{ .@"2d" = ctx };
     }
 
     if (std.mem.eql(u8, context_type, "webgl") or std.mem.eql(u8, context_type, "experimental-webgl")) {
+        // Per spec: return null if a different context type was already requested.
+        if (self._context_type != .none and self._context_type != .webgl) return null;
+
         const ctx = try page._factory.create(WebGLRenderingContext{});
+        self._context_type = .webgl;
         return .{ .webgl = ctx };
     }