feat: verify worker compatibility and add `examples/worker/`

[kalwalt]

Why

@ar-js-org/arjs-plugin-artoolkit runs detection off the main thread ("Web Worker-based detection", "ImageBitmap support — zero-copy frame transfer"). If artoolkit5-ts is to replace artoolkit5-js as its engine, it must be provably usable inside a Worker.

src/ is already DOM-free, which is necessary but not sufficient — WASM instantiation, locateFile resolution, and fetch of .patt / camera_para.dat all need to work in worker scope, and none of that has been tested.

Scope

No worker ships in the library. The target consumer already owns its worker; shipping ours would duplicate it and add versioned surface area for no gain (D6). This issue delivers proof and documentation only.

Deliverable

examples/worker/ where:

• the worker calls createARToolKitState, loadPatternMarker, trackMarker, processFrame
• the main thread only captures frames and renders
• pixel buffers are transferred, not copied
• poses come back as plain { id, matrixGL } objects

Plus a README section documenting the pattern, including the wasmUrl resolution difference in worker scope.

Acceptance

• Worker example tracks a marker with rendering on the main thread
• No DOM API reachable from any src/ code path
• Worker caveats documented
• If WASM init fails in worker scope, findings recorded and an upstream artoolkit5-wasm issue opened