Add atlas-spec loader with duplicate handling

loadAtlasDirs walks one level deep into each --atlas-dir, picks up
*/tiles.atlas.json, and builds bidirectional name/id maps. On
duplicate atlas_id across paths the first registration wins and a
warning string is returned (callers decide whether to print).

src/atlas-loader.js

@@ -0,0 +1,65 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+
+function loadAtlasSpec(specPath) {
+  const raw = fs.readFileSync(specPath, 'utf8');
+  let parsed;
+  try {
+    parsed = JSON.parse(raw);
+  } catch (e) {
+    throw new Error(`${specPath}: invalid JSON (${e.message})`);
+  }
+  if (typeof parsed.atlas_id !== 'string' || parsed.atlas_id.length === 0) {
+    throw new Error(`${specPath}: missing or non-string atlas_id`);
+  }
+  if (!Array.isArray(parsed.tiles)) {
+    throw new Error(`${specPath}: missing tiles[]`);
+  }
+  const tilesByName = {};
+  const tilesById = {};
+  for (const tile of parsed.tiles) {
+    if (typeof tile.id !== 'number' || !Number.isInteger(tile.id)) {
+      throw new Error(`${specPath}: tile missing integer 'id'`);
+    }
+    if (typeof tile.name !== 'string' || tile.name.length === 0) {
+      throw new Error(`${specPath}: tile id=${tile.id} missing 'name'`);
+    }
+    tilesByName[tile.name] = tile.id;
+    tilesById[tile.id] = tile.name;
+  }
+  return { atlas_id: parsed.atlas_id, tilesByName, tilesById, specPath };
+}
+
+function loadAtlasDirs(dirs) {
+  const registry = {};
+  const warnings = [];
+  for (const dir of dirs) {
+    if (!fs.existsSync(dir)) {
+      throw new Error(`no such directory: ${dir}`);
+    }
+    const stat = fs.statSync(dir);
+    if (!stat.isDirectory()) {
+      throw new Error(`not a directory: ${dir}`);
+    }
+    const entries = fs.readdirSync(dir, { withFileTypes: true });
+    for (const entry of entries) {
+      if (!entry.isDirectory()) continue;
+      const specPath = path.join(dir, entry.name, 'tiles.atlas.json');
+      if (!fs.existsSync(specPath)) continue;
+      const spec = loadAtlasSpec(specPath);
+      if (registry[spec.atlas_id]) {
+        warnings.push(
+          `duplicate atlas_id '${spec.atlas_id}' at ${specPath}; ` +
+          `keeping earlier registration from ${registry[spec.atlas_id].specPath}`
+        );
+        continue;
+      }
+      registry[spec.atlas_id] = spec;
+    }
+  }
+  return { registry, warnings };
+}
+
+module.exports = { loadAtlasDirs, loadAtlasSpec };