fix(canvas): harden Canvas2D texture, memory, and clipping paths

[chiefcll]

• CanvasTexture: capture textureData before the first await and abort the load if the texture is freed in-flight, preventing a state/image desync where state=loaded but image=undefined
• CanvasTexture.getImage: return null instead of asserting on a missing image; CanvasRenderer skips the draw for that frame
• Texture: exclude textures in the 'loading' state from cleanup and null out ctxTexture after free() so a reload re-creates it instead of reusing a stale reference
• ImageTexture/CanvasTexture: fail fast for compressed textures in canvas mode (before any fetch/decode) with a descriptive failed state
• TextureMemoryManager/Stage: treat criticalThreshold=0 as disabled and apply it automatically for the canvas renderer, since JS-heap textures are GC-managed and threshold-based upload blocking does not apply
• CanvasRenderer: gate clipping on clippingRect.valid and skip rendering a node clipped to zero area, which previously rendered with no clip at all

Also adds a --renderMode (webgl|canvas|all) flag to the visual-regression runner, storing canvas snapshots in a separate -canvas subdirectory.