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=[]