Fix Text effect font-cache race to allow deterministic renders

[derwin12]

• Headless renders of a sequence with Text effects produced different .fseq output on every run (and could crash) due to a data race in FontManager::init() — its static font/name vectors were lazily
  populated behind a plain bool check with no synchronization, so concurrent frame-parallel render threads could race on the same push_back calls the first time a Text effect rendered.
• Reproduced with back-to-back --headless renders of the same .xsq, --fseqcmp, and XL_VERIFY_STATELESS=1 (captured an ACCESS_VIOLATION inside FontManager::init under load).
• Fix: guard FontManager::init() and FontManager::get_font_names() with std::call_once so the one-time population is properly serialized. No behavioral/data changes — same fonts, same order.