docs(wasm): add build_wasm.sh to document wasm-pack invocation

Captures the exact wasm-pack build command needed to regenerate
solitaire_server/web/pkg/. Removes wasm-pack's package.json and
.gitignore artifacts from the output dir since we manage it directly.
Includes a dependency guard and install instructions.

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

build_wasm.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Rebuild the solitaire_wasm crate and install the output into
+# solitaire_server/web/pkg/ so the server can serve the replay viewer.
+#
+# Prerequisites:
+#   cargo install wasm-pack
+#   rustup target add wasm32-unknown-unknown
+#
+# Run from the repo root:
+#   ./build_wasm.sh
+#
+# The generated files (solitaire_wasm.js + solitaire_wasm_bg.wasm) are
+# committed to git so self-hosters who don't touch the WASM crate can
+# skip this step.  Regenerate after any change to solitaire_wasm/ or
+# solitaire_core/.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+OUT_DIR="$REPO_ROOT/solitaire_server/web/pkg"
+
+if ! command -v wasm-pack &> /dev/null; then
+    echo "error: wasm-pack not found." >&2
+    echo "  Install with: cargo install wasm-pack" >&2
+    exit 1
+fi
+
+echo "Building solitaire_wasm (target: web)..."
+wasm-pack build \
+    --target web \
+    --out-dir "$OUT_DIR" \
+    --no-typescript \
+    "$REPO_ROOT/solitaire_wasm"
+
+# wasm-pack writes a package.json and .gitignore into the output dir.
+# Remove them — we manage the output directory ourselves.
+rm -f "$OUT_DIR/package.json" "$OUT_DIR/.gitignore"
+
+echo "Done. Output:"
+ls -lh "$OUT_DIR"