Add WebAssembly build support via Emscripten

- Cargo.toml: gate `wayland` feature behind cfg(not(wasm32)) so the
  dependency resolves correctly for both native and web targets
- .cargo/config.toml: WASM linker flags (ASYNCIFY, GLFW, memory growth)
- build_web.sh: sets required EMCC_CFLAGS, builds, copies artefacts to web/;
  accepts --serve to start a local HTTP server
- web/index.html: minimal Emscripten shell with canvas and status line
- .gitignore: exclude generated web/*.js and web/*.wasm artefacts
- README: document web build prerequisites and usage

ASYNCIFY lets the native while-loop main loop run unchanged in the browser.
Build output: ~267 KB JS + ~458 KB WASM.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -1 +1,3 @@
 /target
+web/*.js
+web/*.wasm