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>

Cargo.toml

@@ -5,8 +5,14 @@ edition = "2024"
 
 [dependencies]
 rand = "0.10.0"
+
+# wayland is a Linux display-server feature; excluded for WASM builds.
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 raylib = { version = "5.5.1", features = ["wayland"] }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+raylib = { version = "5.5.1" }
+
 [profile.release]
 opt-level = 3
 lto = "thin"