Noto Sans Thai and its siblings have no Latin letters or digits, so on a
machine whose only Thai fonts are Noto no face covers the test run and
fallback_for_run rightly returns None. The guard now asks whether the Thai
substitute covers the whole run, which doesn't go through fallback_for_run.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CX1hMKX28764Lrx5moxs1o