Add native WebAssembly backend

[jserv]

This introduces a lightweight WASM backend without SDL dependency, achieving 60% binary size reduction (891K→357K) compared to SDL-based approach.

• Add backend/wasm.c: Native WASM backend using Canvas 2D API directly
  • Direct Canvas rendering via EM_ASM (no SDL overhead)
  • Browser-native image decoding (JPEG/PNG)
  • Optimized for minimal runtime with MINIMAL_RUNTIME=1
  • Framebuffer accessor functions for zero-copy updates
• Remove twin_dispatch() from API and implementation
  • Eliminated unused function across all backends
  • All backends now use twin_dispatch_once() consistently
• Make SDL backend native-only
  • Removed Emscripten-specific code paths from backend/sdl.c
  • SDL now exclusively for native development/testing

---

Summary by cubic

Added a native WebAssembly backend that renders directly to Canvas 2D, removing SDL from web builds and cutting binary size by 60% (891K→357K). Unified the event loop to twin_dispatch_once and made the SDL backend native-only.

• New Features

  • Native WASM backend (backend/wasm.c) with direct Canvas 2D rendering and browser JPEG/PNG decoding.
  • Framebuffer accessor functions for zero-copy JS updates (MadoCanvas.updateCanvas).
  • Emscripten minimal runtime flags (MINIMAL_RUNTIME=1, emmalloc, no FS) for smaller builds.

• Migration

  • WebAssembly builds must use CONFIG_BACKEND_WASM; SDL is no longer supported with emcc.
  • Remove twin_dispatch; use twin_dispatch_once (or twin_run).
  • Assets switch to demo-wasm(.js/.wasm) with mado-wasm.js; update references and scripts (serve-wasm.py).

[cubic-dev-ai]

2 issues found across 11 files

Prompt for AI agents (all 2 issues)

Understand the root cause of the following 2 issues and fix them.


<file name="assets/web/mado-wasm.js">

<violation number="1" location="assets/web/mado-wasm.js:131">
The bitmask mapping for right and middle mouse buttons in `getButtonState` is incorrect. Browser `e.buttons` uses `2` for right and `4` for middle, but the code maps `2` to Mado&#39;s middle (`4`) and `4` to Mado&#39;s right (`2`). This causes incorrect button state information to be sent to `backend/wasm.c` during `mousemove` events, leading to buggy drag behavior with right and middle clicks.</violation>
</file>

<file name="scripts/serve-wasm.py">

<violation number="1" location="scripts/serve-wasm.py:67">
This file check for `demo-wasm` is incorrect and will fail. The `Makefile` configures Emscripten to generate `demo-wasm.js`, and `assets/web/index.html` correctly includes `&lt;script src=&quot;demo-wasm.js&quot;&gt;`. The development server script is inconsistent with the build output and web page, breaking the local testing workflow.</violation>
</file>

_{React with 👍 or 👎 to teach cubic. Mention @cubic-dev-ai to give feedback, ask questions, or re-run the review.}