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).

tests/atlas-loader.test.js

@@ -0,0 +1,63 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const path = require('node:path');
+const { loadAtlasDirs } = require('../src/atlas-loader');
+
+const FIX = path.join(__dirname, 'fixtures', 'atlases');
+const FIX_DUP = path.join(__dirname, 'fixtures', 'atlases-dup');
+
+test('loadAtlasDirs: discovers two atlases in one dir', () => {
+  const { registry, warnings } = loadAtlasDirs([FIX]);
+  assert.equal(warnings.length, 0);
+  assert.deepEqual(Object.keys(registry).sort(), ['atlas_a', 'atlas_b']);
+});
+
+test('loadAtlasDirs: builds tilesByName and tilesById maps', () => {
+  const { registry } = loadAtlasDirs([FIX]);
+  const a = registry['atlas_a'];
+  assert.equal(a.tilesByName['grass'], 1);
+  assert.equal(a.tilesByName['stone'], 2);
+  assert.equal(a.tilesById[1], 'grass');
+  assert.equal(a.tilesById[2], 'stone');
+  assert.ok(a.specPath.endsWith('tiles.atlas.json'));
+});
+
+test('loadAtlasDirs: empty input → empty registry, no warnings', () => {
+  const { registry, warnings } = loadAtlasDirs([]);
+  assert.deepEqual(registry, {});
+  assert.deepEqual(warnings, []);
+});
+
+test('loadAtlasDirs: duplicate atlas_id → first wins, warning emitted', () => {
+  const { registry, warnings } = loadAtlasDirs([FIX, FIX_DUP]);
+  // First dir wins: atlas_a has 'grass' name, not 'ALT_grass'
+  assert.equal(registry['atlas_a'].tilesByName['grass'], 1);
+  assert.equal(registry['atlas_a'].tilesByName['ALT_grass'], undefined);
+  assert.equal(warnings.length, 1);
+  assert.match(warnings[0], /atlas_a/);
+  assert.match(warnings[0], /duplicate/i);
+});
+
+test('loadAtlasDirs: reverse path order swaps the winner', () => {
+  const { registry, warnings } = loadAtlasDirs([FIX_DUP, FIX]);
+  assert.equal(registry['atlas_a'].tilesByName['ALT_grass'], 1);
+  assert.equal(registry['atlas_a'].tilesByName['grass'], undefined);
+  assert.equal(warnings.length, 1);
+});
+
+test('loadAtlasDirs: missing directory throws', () => {
+  assert.throws(
+    () => loadAtlasDirs(['/no/such/path/exists']),
+    /no such directory/i
+  );
+});
+
+test('loadAtlasDirs: dir with no tiles.atlas.json subdirs → empty registry', () => {
+  // Use sporel-tool-mapper/src/ which has no atlas specs
+  const noAtlasDir = path.join(__dirname, '..', 'src');
+  const { registry, warnings } = loadAtlasDirs([noAtlasDir]);
+  assert.deepEqual(registry, {});
+  assert.equal(warnings.length, 0);
+});