TrailStore, background sampling, auto-pause and the pre-shutdown
auto-save move from UITask to ui-core/TrailEngine.h. TrailScreen still
drives the store directly. The 512-point store now lives on the heap
with the rest of UiCore (static RAM -8 KB on Wio L1, same total).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>