polish: discovery hardening per code review (review round 2 minors)

- Fail fast with clear RuntimeError when themes/ directory is missing
- Document underscore-prefix skip rule
- Tests: broken-manifest/mismatch/no-manifest skip case, missing-dir fail-fast
- tests/test_theme_pro_v3.py: drop fourth hardcoded list, import from core

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/test_theme_pro_v3.py

@@ -6,8 +6,7 @@ import zipfile
 from pathlib import Path
 from bs4 import BeautifulSoup
 from test_factory_v2 import factory, site, zip_theme
-
-THEMES=('automotive','industrial','corporate','minimal')
+from app.core import THEMES
 
 def test_four_complete_theme_families_have_distinct_markup(factory):
     main=[]

tests/test_theme_registry.py

@@ -14,5 +14,26 @@ def test_discover_returns_builtin_themes_with_manifests():
         assert manifest.get('id') == tid
 
 def test_three_whitelists_agree():
+    # Tripwire: the three names must stay the same object. Fails only if someone
+    # reintroduces a hardcoded copy of the list anywhere.
     assert set(core.THEMES) == set(theme_pro.THEMES) == set(theme_store.BUILTIN)
     assert isinstance(core.THEMES, tuple)
+
+def test_discover_skips_broken_theme_dirs(tmp_path, monkeypatch):
+    themes = tmp_path / 'themes'
+    good = themes / 'good-one'; good.mkdir(parents=True)
+    (good / 'manifest.json').write_text(json.dumps({'id': 'good-one'}), encoding='utf-8')
+    broken = themes / 'broken'; broken.mkdir()
+    (broken / 'manifest.json').write_text('{not json', encoding='utf-8')
+    mismatch = themes / 'mismatch'; mismatch.mkdir()
+    (mismatch / 'manifest.json').write_text(json.dumps({'id': 'other-name'}), encoding='utf-8')
+    no_manifest = themes / 'no-manifest'; no_manifest.mkdir()
+    underscore = themes / '_shared'; underscore.mkdir()
+    monkeypatch.setattr(core, 'ROOT', tmp_path)
+    assert core.discover_themes() == ('good-one',)
+
+def test_discover_fails_fast_when_themes_dir_missing(tmp_path, monkeypatch):
+    monkeypatch.setattr(core, 'ROOT', tmp_path)
+    import pytest
+    with pytest.raises(RuntimeError, match='themes directory missing'):
+        core.discover_themes()