Replace convertJSArrayToNumberVector with direct WASM frame buffer

[kalwalt]

Summary

• Eliminates per-frame heap copy: convertJSArrayToNumberVector<uint8_t>() walked the JS typed array element-by-element and created a fresh std::vector on every frame. For a 1280×720 RGBA frame that's ~3.7 MB allocated and freed on every call.
• New approach (Approccio 1): C++ allocates a persistent uint8_t* buffer once via initFrameBuffer(colorSpace), exposes its WASM linear-memory byte offset via getFrameBufferPtr(), and JS writes video data directly with HEAPU8.set() before calling processFrame.
• Lazy-init safety net: processFrame in WebARKitController.js auto-calls initFrameBuffer on first use if not called explicitly, so existing callers keep working without changes.
• All examples updated: worker_threejs.js, worker.js, worker_bufferCopy_threejs.js, worker_jsfeatNext.js all call initFrameBuffer explicitly with the correct colorspace after tracker load.

Changes

File	Change
emscripten/WebARKitJS.h	Add initFrameBuffer, getFrameBufferPtr, destructor, private buffer members; update processFrame signature
emscripten/WebARKitJS.cpp	Implement buffer allocation, bppForColorSpace helper, null guard, destructor
emscripten/bindings.cpp	Expose initFrameBuffer and getFrameBufferPtr to JS via embind
src/WebARKitController.js	Add initFrameBuffer wrapper, lazy-init + HEAPU8.set() in processFrame
examples/worker_threejs.js	Explicit initFrameBuffer(RGBA)
examples/worker.js	Explicit initFrameBuffer(GRAY)
examples/worker_bufferCopy_threejs.js	Explicit initFrameBuffer(RGBA)
examples/worker_jsfeatNext.js	Explicit initFrameBuffer(GRAY)
build/, dist/	Rebuilt artifacts

Pitfalls addressed

• Destructor frees the buffer to avoid WASM heap leak on teardown
• HEAPU8 is read fresh each frame (not cached) to survive ALLOW_MEMORY_GROWTH heap relocations
• initFrameBuffer colorspace must match processFrame colorspace — all examples verified and corrected (found and fixed a GRAY/RGBA mismatch in worker_jsfeatNext.js)
• initTrackerGray intentionally left unchanged (one-time call, copy overhead negligible)

Test plan

• npm run build-docker — compiled cleanly, no new warnings
• npm run build-es6 — webpack bundled successfully
• Open examples/teblid_example.html or threejs_worker_ES6.js in browser and verify marker tracking still works

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