ci: add nvim.wasm build and nightly release

Add a dedicated nvim.wasm build workflow and
include the resulting WASM artifact in nightly releases.

.github/scripts/build_wasm.sh

@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+
+# Build nvim.wasm
+
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+OUTDIR="$ROOT/build-wasm"
+
+source "$ROOT/.emsdk/emsdk_env.sh"
+
+mkdir -p "$OUTDIR"
+
+if ! command -v emcc >/dev/null 2>&1; then
+  echo "ERROR: emcc not found"
+  exit 1
+fi
+
+if ! command -v zig >/dev/null 2>&1; then
+  echo "ERROR: zig not found"
+  exit 1
+fi
+
+emcc --version
+zig version
+
+# Install Binaryen if it is not already available.
+BINARYEN_VERSION=132
+BINARYEN_DIR="$ROOT/.binaryen"
+BINARYEN_BIN="$BINARYEN_DIR/binaryen-version_${BINARYEN_VERSION}/bin"
+
+if [ ! -x "$BINARYEN_BIN/wasm-opt" ]; then
+  echo "Installing Binaryen ${BINARYEN_VERSION}..."
+
+  mkdir -p "$BINARYEN_DIR"
+
+  curl -L \
+    "https://github.com/WebAssembly/binaryen/releases/download/version_${BINARYEN_VERSION}/binaryen-version_${BINARYEN_VERSION}-x86_64-linux.tar.gz" \
+    -o /tmp/binaryen.tar.gz
+
+  tar -xf /tmp/binaryen.tar.gz -C "$BINARYEN_DIR"
+fi
+
+export PATH="$BINARYEN_BIN:$PATH"
+
+wasm-opt --version
+
+zig build nvim_bin \
+  -Dtarget=wasm32-emscripten \
+  -Dcpu=generic+atomics+bulk_memory+mutable_globals \
+  -Demscripten-sysroot="$EMSDK/upstream/emscripten/cache/sysroot" \
+  -Doptimize=ReleaseSmall
+
+ZIG_OUT="$ROOT/zig-out/bin"
+
+# Optimize the final WASM artifact with Binaryen.
+wasm-opt -Oz -o "$ZIG_OUT/nvim.wasm" "$ZIG_OUT/nvim.wasm"
+
+required=(
+  nvim.wasm
+  nvim.js
+  nvim.data
+)
+
+missing=()
+
+for name in "${required[@]}"; do
+  if [ -f "$ZIG_OUT/$name" ]; then
+    cp "$ZIG_OUT/$name" "$OUTDIR/$name"
+  else
+    missing+=("$name")
+  fi
+done
+
+if [ "${#missing[@]}" -ne 0 ]; then
+  echo "ERROR: Missing required WASM artifacts: ${missing[*]}"
+  ls -la "$ZIG_OUT" || true
+  exit 2
+fi
+
+echo "build-wasm.sh finished successfully."