Scaffold sporel-tool-mapper package

Empty Node 18+ CommonJS package with bin entry and node --test
script. Subcommand implementations follow in subsequent commits.

package.json

@@ -0,0 +1,16 @@
+{
+  "name": "sporel-tool-mapper",
+  "version": "0.1.0",
+  "description": "CLI for v2 Sporel map files: encode/decode packed-u32 GIDs, build maps from a text DSL, inspect stats.",
+  "bin": {
+    "sporel-mapper": "bin/mapper.js"
+  },
+  "scripts": {
+    "test": "node --test tests/*.test.js"
+  },
+  "engines": {
+    "node": ">=18"
+  },
+  "license": "UNLICENSED",
+  "private": true
+}