Init atlas-baker tool with source-dir scan and pair-detection

Creates the sporel-tool-atlas-baker Node CLI scaffold and the first
phase of the bake pipeline: scan a source directory for *_diffuse.png
files, pair them with optional *_height.png companions, and return
alphabetically-sorted sources for deterministic re-bakes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

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+# sporel-tool-atlas-baker
+
+Builds paired diffuse + height PNG atlases for the Sporel map-lib v0.3.0+ format.
+
+## Usage
+
+```bash
+sporel-atlas-baker \
+  --in <source-dir> \
+  --out <atlas-dir> \
+  --atlas-id <stable-id> \
+  [--tile-size 64] \
+  [--max-size 4096] \
+  [--lock <existing-lock.json>] \
+  [--blocks-sight-pattern <regex>]
+```
+
+See `sporel-meta/docs/superpowers/specs/2026-05-21-map-multi-layer-design.md` §4 for the format spec.
+
+## Development
+
+```bash
+npm install
+npm test
+```