feat(ui): GPS breadcrumb phase 1 — storage, sampling, Summary view

Adds Tools › Breadcrumb. Enter on the screen toggles tracking; while
active, UITask::loop samples GPS at the cadence chosen in settings
(default 1 min) and drops the point into a 256-entry RAM ring guarded
by a min-distance gate (default 25 m). A "G" indicator joins "A" in
the status bar with the same blink convention.

New storage:
- examples/companion_radio/Breadcrumb.h — BreadcrumbStore class
- 256 × 12 B = 3 KB RAM (survives auto-off, lost on reboot — explicit
  flash-slot save comes in phase 4)
- Haversine min-delta gate, total distance, elapsed seconds, current
  km/h, bounding-box helper

New screen:
- examples/companion_radio/ui-new/BreadcrumbScreen.h — Summary view
  with Status, Points, Dist (m or km), Time (h:mm), Speed; Enter
  toggles tracking, Esc returns to Tools

Schema bump 0xC0DE0002 → 0xC0DE0003 for two new NodePrefs bytes:
breadcrumb_interval_idx (1min default) and breadcrumb_min_delta_idx
(25m default). Older saves leave both at zero, which matches the new
defaults. DataStore::loadPrefsInt/savePrefs read/write the pair.

Logging on/off is a runtime flag inside BreadcrumbStore — not a
preference — so reboot returns to the off state cleanly; the user
will be able to persist a snapshot to a slot in phase 4.

Phase 2 (map view), 3 (point list), 4 (slot save/load, GPX export over
USB), and 5 (settings UI) follow incrementally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

FEATURES.md

@@ -58,12 +58,19 @@ Render layout (250×122 landscape e-ink):
 
 Joystick navigation is natural with 6 tiles (UP/DOWN between rows, LEFT/RIGHT within row).
 
-### 📋 GPS breadcrumb
-
-📋 planned
+### 🚧 GPS breadcrumb
 
 Tools › Breadcrumb. Periodically samples `(lat, lon, ts)` into a RAM ring buffer; user explicitly saves snapshots to flash.
 
+**Logging is a runtime state**, not a settings value. User starts/stops from the
+Tools › Breadcrumb screen. Once active, sampling continues in the background
+regardless of which screen is shown, and a `G` indicator appears in the status
+bar (analogous to `A` for auto-advert). A reboot resets the active state to
+off; the RAM trail is also lost on reboot unless saved to a flash slot first.
+
+Settings only control sampling cadence and the min-distance gate — they don't
+enable/disable the feature.
+
 **Storage model — RAM ring with explicit save**
 
 Rationale: auto-off only blanks the display, the firmware keeps running, so the RAM trail survives every idle scenario. Typical use is a single trip start→stop while wearing the device; persisting across reboots is rarely wanted. RAM-only avoids ~1400 flash writes/day and the LittleFS wear that comes with continuous logging.
@@ -98,8 +105,9 @@ Statistics computed on the fly walking the ring:
 - Current speed: dist(last, prev) / (ts[last] - ts[prev])
 
 Settings:
-- Settings › GPS › Breadcrumb interval: off / 30 s / 1 min / 5 min / 15 min (default 1 min)
+- Settings › GPS › Breadcrumb interval: 30 s / 1 min / 5 min / 15 min (default 1 min) — only the cadence; logging on/off is a Tools toggle
 - Settings › GPS › Breadcrumb min delta: 5 m / 25 m / 100 m (skip near-stationary samples to keep the ring densely populated with real movement)
+- Export format: GPX (standard for GPS tracks; OSMAnd / Garmin compatible)
 
 Schema impact: new prefs fields `uint8_t breadcrumb_interval_idx`, `uint8_t breadcrumb_min_delta_idx`. Sentinel bump. The slot files are separate from prefs.