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>

tests/scan.test.js

@@ -0,0 +1,28 @@
+// tests/scan.test.js
+const { test } = require('node:test');
+const assert = require('node:assert');
+const path = require('node:path');
+const { scanSourceDir } = require('../src/scan');
+
+const FIXTURE_DIR = path.resolve(__dirname, 'fixtures/small-fresh');
+
+test('scan: 3 diffuse + 1 paired height', () => {
+    const { sources, errors } = scanSourceDir(FIXTURE_DIR);
+    assert.deepStrictEqual(errors, []);
+    assert.strictEqual(sources.length, 3);
+    // Sorted alphabetically
+    assert.strictEqual(sources[0].name, 'grass');
+    assert.strictEqual(sources[1].name, 'stone');
+    assert.strictEqual(sources[2].name, 'water');
+    // grass + stone have no height
+    assert.strictEqual(sources[0].heightPath, null);
+    assert.strictEqual(sources[1].heightPath, null);
+    // water has height
+    assert.ok(sources[2].heightPath?.endsWith('water_height.png'));
+});
+
+test('scan: empty dir -> error', () => {
+    const { sources, errors } = scanSourceDir(path.resolve(__dirname, 'fixtures/_empty'));
+    assert.strictEqual(sources.length, 0);
+    assert.ok(errors.length > 0);
+});