Phase 3. Fixed 16-entry table of (lat, lon, ts, label[12]) persisted to
/waypoints with a magic+version header (mirrors TrailStore's format).
Unlike the RAM-only trail, waypoints are loaded in UITask::begin() and
rewritten on change via UITask::saveWaypoints(), so they survive reboots.
add/remove/rename/clear operate on the in-RAM table; the screen layer
(phase 5) drives persistence after each edit.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
