State-pattern reference migration per GUIDE-RJS-004: render never
paints; onStateChange is the only DOM site; render_timestamp
forcing key re-resolves nav links on every navigation; null
sentinel for theme so the localStorage restore always repaints;
dark-mode toggle goes through changeState. Removed four unused
legacy Handlebars templates (and their ids + the handlebars.js
dependency).

Gotchas documented in CLAUDE.md: {mutex: 'changestate'} on a
method that calls changeState deadlocks (mutex not reentrant);
loopEventListener prevent_default defaults to true.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>