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>

build_web.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# Build the endless runner for WebAssembly via Emscripten.
+# Usage: ./build_web.sh [--serve]
+set -euo pipefail
+
+# Flags required by raylib-sys for WASM compilation.
+export EMCC_CFLAGS="-O3 -sUSE_GLFW=3 -sASSERTIONS=1 -sWASM=1 -sASYNCIFY -sGL_ENABLE_GET_PROC_ADDRESS=1"
+
+echo "Building for wasm32-unknown-emscripten..."
+cargo build --target wasm32-unknown-emscripten --release
+
+OUT=target/wasm32-unknown-emscripten/release
+
+echo "Copying build artefacts to web/..."
+cp "$OUT/endless_runner.js"   web/
+cp "$OUT/endless_runner.wasm" web/
+
+echo ""
+echo "Build complete. Output in web/"
+
+if [[ "${1-}" == "--serve" ]]; then
+  echo "Serving at http://localhost:8080"
+  python3 -m http.server 8080 --directory web
+fi