fix(🌐): keep the web canvas usable when its layout effect re-runs

[dennytosp]

Fixes #3976.

WebGLRenderer.dispose() calls WEBGL_lose_context.loseContext() on the <canvas> element. Per the spec that loses the element's context permanently — a later getContext("webgl2") returns the same dead context object, and only restoreContext() revives it.

But the renderer is created and disposed inside a useLayoutEffect, and effect cleanup does not mean the element is going away. React re-runs layout effects on a preserved host node in at least two everyday cases:

• StrictMode's DEV double-invoke, on every mount in development;
• reappearLayoutEffects when a hidden Activity/offscreen subtree is revealed — which is how React Navigation and Expo Router keep inactive routes mounted.

The second construction then hands CanvasKit a dead canvas and faults inside wasm with Cannot read properties of null (reading 'rangeMin'), taking the tree down.

What #3929 actually needed is CanvasKit.deleteContext, which unregisters the context from CanvasKit's registry and drops its reference to the detached element — that stays. loseContext() only released the drawing buffer eagerly, and it was the sole part of dispose() that left the element unusable. StaticWebGLRenderer.cleanupRenderResult() keeps its call: that one is on a throwaway OffscreenCanvas that is never reused.

The new test in SkiaPictureView.web.spec.tsx drives the component through StrictMode with a WebGL context mock that models the spec'd behaviour — once lost, the element's context stays lost, and GetWebGLContext returns no handle for it. On main it fails with the reported stack:

Could not create a WebGL context
  at new WebGLRenderer (src/views/SkiaPictureView.web.tsx:48:13)
  at commitHookLayoutEffects
  at reappearLayoutEffects
  at doubleInvokeEffectsOnFiber
  at recursivelyTraverseAndDoubleInvokeEffectsInDEV

The mock is wired into the existing getContext stub, so the four pre-existing tests now exercise a real context object instead of null; they still pass unchanged.