[1a] Replace EM_ASM_ block in ARimageFsetDisplay.cpp with embind

[kalwalt]

Summary

Replace EM_ASM_ block in emscripten/ARimageFsetDisplay.cpp (lines 180–190) with idiomatic C++ / embind

Environment

• Product/Service: FeatureSET-Display — Emscripten/wasm native layer
• File: emscripten/ARimageFsetDisplay.cpp lines 180–190, emscripten/bindings.cpp lines 46–55

Problem Description

The current block uses EM_ASM_ to write frameIbwpointer, frameimgBWsize, and frameFeaturePoints onto a JS-side arfset.frameMalloc object. This incurs an extra wasm→JS crossing on every frame and introduces hidden global state. The same three values are already exposed through the embind nftMarker value_object (pointer, imgBWsize, nftFeaturePoints in bindings.cpp:46-55), so JS can read them directly from the returned struct.

Expected Behavior

No EM_ASM_ blocks remain in the hot path. JS reads pointer, imgBWsize, and nftFeaturePoints directly from the nftMarker struct returned by embind, eliminating the side-channel global.

Actual Behavior

EM_ASM_ fires every frame, crossing the wasm↔JS boundary and mutating arfset.frameMalloc as a hidden side-channel.

Tasks

• Pick approach (recommended: drop EM_ASM_ and read from nftMarker)
• Remove the EM_ASM_ block from ARimageFsetDisplay.cpp:180-190
• Update JS callers in js/arfset.api.js and src/ARFset.js to read from the returned nftMarker struct instead of arfset.frameMalloc

Impact

Medium — Performance: eliminates a per-frame wasm→JS crossing and removes hidden global state. No functional regression expected.

Additional Context

If a JS-visible global is genuinely required, prefer emscripten::val (pure C++, type-safe):

val::global("arfset")["frameMalloc"].set("frameIbwpointer", arc->imgBW);

Last resort: EM_JS(void, set_frame_malloc, (...), { ... }) — parsed once at build time.

See also: issue 1b (listener leak fix) should land first as it touches the same drawing path.