Scaffold sporel-tool-mapper package

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

README.md

@@ -0,0 +1,16 @@
+# 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.)