wasm: introduce wasm backend

[jcelerier]

Adds a WebAssembly / web backend (include/avnd/binding/wasm/) that compiles a single Avendish plug-in to a small, self-contained ES module — without dragging in libossia/score. The same plug-in source that targets pd/vst3/max now also targets the web.

Delivery modes

• Browser standalone — a self-contained page (<c_name>.html) that sets up an AudioContext + AudioWorklet and an auto-generated UI.
• Web Audio Module 2 (WAM2) — a standards-compliant WAM package (wam/<c_name>/) loadable by web DAWs.
• Node / headless — the same web,worker,node ES module runs in Node/Deno/Bun for offline rendering.

What's bridged

• Audio: manual AudioWorkletProcessor + planar-heap marshaling; float/double conversion, mono↔poly, multichannel / variable-channel negotiation.
• Controls & value I/O: every Avendish parameter type (float/int/bool/enum/string/xy/xyz/rgba/range/multislider/list/map/variant/…) exposed via Embind with getParameterInfo/set/get (+ normalized); value inputs and outputs surfaced separately in the UI.
• MIDI (Web MIDI in/out), messages (named RPC, JS→plugin), callbacks (plugin→JS, queued + drained), sample-accurate controls.
• UI: a painter→CanvasRenderingContext2D adapter for custom immediate-mode draw items, plus an auto-UI from the introspected layout.
• GPU: not in this PR (texture/buffer consumers are planned; geometry intentionally out of scope).

Build

• cmake/avendish.wasm.cmake adds avnd_make_wasm (wired into avnd_make_object/avnd_make_audioplug), plus wasm_gallery (build all + a generated landing page) and wasm_serve (COOP/COEP dev server) targets.
• Keeps RTTI on (Embind requires it) while disabling exceptions; fmt is forced header-only under Emscripten.

Tests

Headless Node tests cover the ring buffer, heap audio marshaling, the DSP/node runner, control round-trips across all data types, MIDI/messages/callbacks/sample-accurate, the painter→canvas call sequence, the auto-UI DOM, and the WAM2 parameter mapping.

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