fix: resolve canvas texture race conditions

[wouterlucas]

• CanvasTexture.load(): capture textureData ref before first await to prevent freeTextureDataTask microtask from nulling it mid-flight
• CanvasTexture.load(): abort if texture is freed while load is in-flight to prevent state/image desync (state=loaded but image=undefined)
• CanvasTexture.getImage(): return null instead of asserting on missing image, allowing callers to skip gracefully
• CanvasRenderer.renderContext(): guard against null return from getImage()
• Texture.free(): null out ctxTexture after freeing to prevent stale reference reuse on reload
• Texture.canBeCleanedUp(): exclude textures in 'loading' state from eviction to prevent cleanup racing an in-flight load