V3-12: crash reporting (code only)

shouldInitializeCrashReporting() and scrubExtra() are pure, fully unit-tested decision/scrubbing functions wired into sentry_flutter via beforeSend/beforeBreadcrumb. Config.crashReportingEnabled defaults off; the DSN is a compile-time --dart-define, not a preference. RecordingEngine gained an injected onUnexpectedStop callback, firing once when restoreAfterProcessDeath finds a trip still 'recording' at launch -- the process died without a user stop.

Marked partially done: the ticket's real acceptance criteria (a forced crash landing in a real Sentry project from a release build, inspecting a real payload) need a Sentry account and a release build this environment can't produce.

docs/v3/README.md

@@ -28,7 +28,7 @@ backup are v4 — see [../BACKLOG.md](../BACKLOG.md).
 | [V3-09](V3-09-route-following.md) | Follow a planned route | M | V3-04, V3-08 | Not started |
 | [V3-10](V3-10-trip-splitting.md) | Trip splitting | S | — | Done |
 | [V3-11](V3-11-offline-tiles.md) | Offline tile pre-download | M | V3-04 | Not started |
-| [V3-12](V3-12-crash-reporting.md) | Crash reporting | S | — | Not started |
+| [V3-12](V3-12-crash-reporting.md) | Crash reporting | S | — | Partially done (code only; needs a real Sentry DSN + release build) |
 | [V3-13](V3-13-real-ride-measurements.md) | Real-ride measurements | M | **riding** | Not started |
 | [V3-14](V3-14-gpx-interop.md) | GPX interoperability | S | V3-01 | Partially done (code only; needs real-device verification) |
 | [V3-15](V3-15-auto-pause.md) | Auto-pause | M | V3-13 *(gated)* | Not started |