fix: ensure WebGL→WASM fallback catches synchronous throws

[Copilot]

Promise.resolve(WebGlVectorBackend.create()) evaluates create() synchronously before the promise is constructed — a synchronous throw escapes the promise chain entirely, making the .catch() WASM fallback dead code.

Change

Wrap the call in .then() so any synchronous throw is captured as a rejection:

// Before — synchronous throw bypasses .catch()
return Promise.resolve(WebGlVectorBackend.create()).catch(() =>
  WasmVectorBackend.create(wasmBytes)
);

// After — throw is converted to a rejected promise, fallback fires correctly
return Promise.resolve()
  .then(() => WebGlVectorBackend.create())
  .catch(() => WasmVectorBackend.create(wasmBytes));

This aligns the WebGL path with how the WebGPU and WebNN paths already handle async creation and fallback.

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[Copilot]

Pull request overview

Fixes the WebGL→WASM fallback in createVectorBackend so that a synchronous throw from WebGlVectorBackend.create() is properly caught and triggers the WASM fallback, rather than escaping the promise chain.

Changes:

• Wraps WebGlVectorBackend.create() inside .then() instead of passing it directly to Promise.resolve(), ensuring synchronous throws become promise rejections.