Scaffold sporel-tool-mapper package

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

.gitignore

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+node_modules/
+.DS_Store
+*.log

README.md

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+# sporel-tool-mapper
+
+Standalone CLI for v2 Sporel map files.
+
+## Status
+
+`v0.1.0` — initial scaffold. See `docs/superpowers/specs/2026-05-23-sporel-tool-mapper-design.md` in `sporel-meta` for the full spec.
+
+## Subcommands
+
+- `encode <atlas_idx> <tile_id> [rotation]`
+- `decode <gid>`
+- `build <spec.txt> <out.json> [--atlas-dir <path> ...]`
+- `inspect <map.json> [--atlas-dir <path> ...]`
+
+(Detailed usage and examples are added in Task 12.)

package.json

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+{
+  "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
+}