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/fixtures/_setup.js

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+// tests/fixtures/_setup.js
+// Run once to create test fixtures. Not part of regular tests.
+const fs = require('node:fs');
+const path = require('node:path');
+const { PNG } = require('pngjs');
+
+function writeSolidPng(filePath, w, h, rgba) {
+    const png = new PNG({ width: w, height: h });
+    for (let y = 0; y < h; y++) {
+        for (let x = 0; x < w; x++) {
+            const i = (y * w + x) * 4;
+            png.data[i + 0] = rgba[0];
+            png.data[i + 1] = rgba[1];
+            png.data[i + 2] = rgba[2];
+            png.data[i + 3] = rgba[3];
+        }
+    }
+    fs.writeFileSync(filePath, PNG.sync.write(png));
+}
+
+function writeSolidL8Png(filePath, w, h, v) {
+    const png = new PNG({ width: w, height: h, colorType: 0 });  // grayscale
+    for (let y = 0; y < h; y++) {
+        for (let x = 0; x < w; x++) {
+            png.data[(y * w + x) * 4 + 0] = v;
+            png.data[(y * w + x) * 4 + 1] = v;
+            png.data[(y * w + x) * 4 + 2] = v;
+            png.data[(y * w + x) * 4 + 3] = 255;
+        }
+    }
+    fs.writeFileSync(filePath, PNG.sync.write(png));
+}
+
+const dir = path.resolve(__dirname, 'small-fresh');
+fs.mkdirSync(dir, { recursive: true });
+writeSolidPng(path.join(dir, 'grass_diffuse.png'),  32, 32, [110, 170,  80, 255]);
+writeSolidPng(path.join(dir, 'stone_diffuse.png'),  32, 32, [120, 120, 120, 255]);
+writeSolidPng(path.join(dir, 'water_diffuse.png'),  32, 32, [ 60, 110, 180, 255]);
+writeSolidL8Png(path.join(dir, 'water_height.png'), 32, 32, 0);   // L8=0 plane
+console.log('fixtures written: small-fresh/');

tests/scan.test.js

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+// 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);
+});