feat: add built-in Map type with String keys

Add Map<String, V> as a first-class built-in type for key-value storage,
needed for self-hosting the compiler (parser/typechecker/interpreter all
rely heavily on hashmaps).

- types.rs: Type::Map(K,V) variant, all match arms (unify, apply, etc.)
- interpreter.rs: Value::Map, 12 BuiltinFn variants (new/set/get/contains/
  remove/keys/values/size/isEmpty/fromList/toList/merge), immutable semantics
- typechecker.rs: Map<K,V> resolution in resolve_type
- js_backend.rs: Map as JS Map with emit_map_operation()
- c_backend.rs: LuxMap struct (linear-scan), runtime fns, emit_map_operation()
- main.rs: 12 tests covering all Map operations
- validate.sh: now checks all projects/ directories too

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

scripts/validate.sh

@@ -63,6 +63,26 @@ for pkg in path frontmatter xml rss markdown; do
     fi
 done
 
+# --- Project checks ---
+PROJECTS_DIR="$(pwd)/projects"
+for proj_dir in "$PROJECTS_DIR"/*/; do
+    proj=$(basename "$proj_dir")
+    if [ -f "$proj_dir/main.lux" ]; then
+        step "lux check (project: $proj)"
+        OUTPUT=$("$LUX" check "$proj_dir/main.lux" 2>&1 || true)
+        if echo "$OUTPUT" | grep -qi "error"; then fail; else ok; fi
+    fi
+    # Check any standalone .lux files in the project
+    for lux_file in "$proj_dir"/*.lux; do
+        [ -f "$lux_file" ] || continue
+        fname=$(basename "$lux_file")
+        [ "$fname" = "main.lux" ] && continue
+        step "lux check (project: $proj/$fname)"
+        OUTPUT=$("$LUX" check "$lux_file" 2>&1 || true)
+        if echo "$OUTPUT" | grep -qi "error"; then fail; else ok; fi
+    done
+done
+
 # --- Summary ---
 printf "\n${BOLD}═══ Validation Summary ═══${NC}\n"
 if [ $FAILED -eq 0 ]; then