feat(companion): GPS averaging for waypoint marking (Mark avg)

Add a "Mark avg" trail setting (Off/5/10/30 s). When set, Tools › Trail ›
Mark here samples the GPS fix once a second for the chosen window and stores
the mean position instead of one instantaneous fix — a steadier, more
accurate mark for a precise spot. A progress screen shows time left + sample
count; Cancel aborts; the window then opens the label keyboard as usual.
Off (default) keeps marking instant.

Sampling runs in WaypointsView::poll() (forwarded from TrailScreen::poll())
so it ticks on the main loop, independent of the slow e-ink render cadence;
int64 accumulators avoid overflow over 30 samples.

Persisted as NodePrefs::gps_avg_idx (schema 0xC0DE0016): struct field +
option table, DataStore rd/write/clamp in lockstep. sizeof unchanged (the
uint8_t fits existing tail padding) so the 2488 tripwire still holds.

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

docs/solo_features/tools_screen/tools_screen.md

@@ -127,6 +127,7 @@ Cycle views with **LEFT / RIGHT**:
 | ---------- | --------- | ------------------------------------------------------- |
 | Min dist   | always    | Sample gate, 4 levels — metric: 5/10/25/100 m, imperial: 15/30/75/300 ft |
 | Auto-pause | always    | Off / 1 / 2 / 5 min — auto-freeze the trail after a stop, resume on movement (see below) |
+| Mark avg   | always    | Off / 5 / 10 / 30 s — GPS averaging for **Mark here** (see Waypoints below) |
 | Readout    | Summary view | Summary shows Speed or Pace (in the global unit system) |
 | Grid       | Map view  | Toggle scale grid on the map                            |
 
@@ -140,6 +141,8 @@ A waypoint is a saved spot — your car, camp, a water source — that you can n
 
 **Dropping a waypoint** — **Hold Enter → Mark here**. This captures the current GPS fix and opens the on-screen keyboard for a short label (up to 11 characters — e.g. `CAR`, `CAMP`, `H2O`). Leaving it blank auto-names it `WP1`, `WP2`, … Marking works whether or not the trail is being recorded; it needs a GPS fix (otherwise it reports *No GPS fix*).
 
+**GPS averaging** — with **Settings → Mark avg** set (5 / 10 / 30 s), *Mark here* doesn't snapshot a single fix; it samples the GPS once a second for that window and stores the **mean** position, for a steadier mark than one instantaneous reading (handy for a precise spot — a cache, a car, a trailhead). A short screen shows the time left and the sample count while it runs; **Cancel** aborts. When the window closes it opens the label keyboard as usual. With **Mark avg = Off** (the default) marking is instant.
+
 **Adding by coordinates** — open **Hold Enter → Waypoints** and select the **+ Add by coords** row (always the last entry in the list). This creates a waypoint without being there — no GPS fix required (handy for a meeting point or a spot read off a map). It opens a small form with three editable rows plus **Save**:
 
 |           OLED             |           E-Ink            |