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>

src/scan.js

@@ -0,0 +1,67 @@
+// src/scan.js
+// Scans a source directory for *_diffuse.{png,webp,jpg,jpeg} files and
+// pairs them with optional *_height.{png,webp,jpg,jpeg} companions.
+//
+// Returns: { sources: Array<{name, diffusePath, heightPath?}>, errors: Array<string> }
+// `name` = filename without "_diffuse.<ext>" suffix.
+
+const fs = require('node:fs');
+const path = require('node:path');
+
+const SUPPORTED_EXT = ['.png', '.webp', '.jpg', '.jpeg'];
+
+function stripSuffix(filename, suffix) {
+    for (const ext of SUPPORTED_EXT) {
+        const full = `${suffix}${ext}`;
+        if (filename.toLowerCase().endsWith(full)) {
+            return filename.slice(0, -full.length);
+        }
+    }
+    return null;
+}
+
+function scanSourceDir(srcDir) {
+    if (!fs.existsSync(srcDir)) {
+        return { sources: [], errors: [`scan: source dir does not exist: ${srcDir}`] };
+    }
+    const entries = fs.readdirSync(srcDir, { withFileTypes: true });
+    const diffuseMap = new Map();   // name -> path
+    const heightMap  = new Map();   // name -> path
+
+    for (const entry of entries) {
+        if (!entry.isFile()) continue;
+        const fn = entry.name;
+        const dName = stripSuffix(fn, '_diffuse');
+        if (dName !== null) {
+            diffuseMap.set(dName, path.join(srcDir, fn));
+            continue;
+        }
+        const hName = stripSuffix(fn, '_height');
+        if (hName !== null) {
+            heightMap.set(hName, path.join(srcDir, fn));
+        }
+    }
+
+    const errors = [];
+    for (const hName of heightMap.keys()) {
+        if (!diffuseMap.has(hName)) {
+            errors.push(`scan: height without paired diffuse: ${hName}`);
+        }
+    }
+
+    // Sort alphabetically by name for determinism
+    const names = [...diffuseMap.keys()].sort((a, b) => a.localeCompare(b));
+    const sources = names.map(name => ({
+        name,
+        diffusePath: diffuseMap.get(name),
+        heightPath:  heightMap.get(name) || null,
+    }));
+
+    if (sources.length === 0) {
+        errors.push(`scan: no *_diffuse.{png,webp,jpg,jpeg} found in: ${srcDir}`);
+    }
+
+    return { sources, errors };
+}
+
+module.exports = { scanSourceDir };