V3-01: activity type per ride

Adds Activity (motorcycle/bicycle/scooter/skateboard/running/walking/other) as
a column on Trip, and threads it through as real behaviour rather than a label:
ActivityProfile (domain/activity_profile.dart) carries a noise floor, accuracy
gate, histogram bucket width, and elevation smoothing window/threshold per
activity, consumed by sanitizeSpeedKmh, isUsableFix, computeSummary, Accumulator
and speedHistogram. The motorcycle profile reproduces the exact constants the
app shipped with before this existed, and a test asserts they never drift apart.

This is the port's first real migration: schemaVersion 1 -> 2,
m.addColumn(trips, trips.activity) with a motorcycle default so every existing
row survives unmodified. Proven with a hand-built v1 SQLite file (raw sqlite3,
not drift_dev's schema tooling) that a real database with real rides upgrades
and keeps every trip, segment and point.

No picker in front of Start: a new trip defaults to whichever activity was most
recently used, derived live from the trips table rather than duplicated into a
Config field. Editable afterwards on trip detail, which recomputes aggregates
under the new profile immediately -- a walking pace that reads as noise under a
motorcycle's floor reads as real movement once the activity is corrected, and
there's a test proving exactly that transition.

The widget-test pass for the activity-picker sheet caught a real overflow bug:
seven options overflowed a Column-based bottom sheet the same way the record
screen once did. Fixed with a scrollable ListView + isScrollControlled, same
shape as that earlier fix.

188 tests (171 -> 188): 2 migration, 5 ActivityProfile, 3 profile-threading
proofs in computeSummary, 5 repository, 2 widget. Analyze clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/src/recording/recording_engine.dart

@@ -37,6 +37,7 @@ import 'dart:async';
 import 'package:synchronized/synchronized.dart';
 
 import '../data/trip_repository.dart';
+import '../domain/activity_profile.dart';
 import '../domain/models.dart';
 import '../telemetry/live_telemetry.dart';
 import '../telemetry/telemetry.dart';
@@ -93,7 +94,14 @@ class RecordingEngine {
   StreamSubscription<LocationFix>? _subscription;
   Timer? _flushTimer;
 
-  final Accumulator _accumulator = Accumulator();
+  /// Rebuilt, not mutated, whenever a new trip session begins — see [start] — so its
+  /// [Accumulator.profile] always matches the trip it is accumulating for.
+  Accumulator _accumulator = Accumulator();
+
+  /// Which activity's defaults are currently in force for the fix path — the accuracy
+  /// gate and noise floor in [_onFix] read this directly, since that callback must stay
+  /// synchronous and cannot query the trip's activity from the database per fix.
+  ActivityProfile _activeProfile = ActivityProfile.motorcycle;
 
   // Read by the fix callback, written by the lifecycle path.
   int _currentTripId = 0;
@@ -132,7 +140,15 @@ class RecordingEngine {
 
     await _writeLock.synchronized(() async {
       if (handle.tripId != _currentTripId) {
-        _accumulator.reset();
+        // A different trip session: rebuild the accumulator under that trip's own
+        // activity profile, rather than mutating the previous trip's accumulator in
+        // place — a ride switched from walking to a motorcycle mid-session must not
+        // keep walking's noise floor.
+        final trip = await _repo.tripById(handle.tripId);
+        _activeProfile = ActivityProfile.forActivity(
+          trip?.activity ?? Activity.motorcycle,
+        );
+        _accumulator = Accumulator(profile: _activeProfile);
         await _seedAccumulator(handle.tripId);
       }
       // A new segment must not measure distance back to the pre-pause position.
@@ -233,8 +249,9 @@ class RecordingEngine {
         // avoids.
         final handle = await _repo.resumeIntoNewSegment(_now());
         if (handle == null) break;
+        _activeProfile = ActivityProfile.forActivity(trip!.activity);
         await _writeLock.synchronized(() async {
-          _accumulator.reset();
+          _accumulator = Accumulator(profile: _activeProfile);
           await _seedAccumulator(handle.tripId);
           _accumulator.onSegmentChanged();
         });
@@ -246,6 +263,7 @@ class RecordingEngine {
         _setState(RecorderState.recording);
       case TripState.paused:
         _currentTripId = trip!.id;
+        _activeProfile = ActivityProfile.forActivity(trip.activity);
         _setState(RecorderState.paused);
       case TripState.completed:
       case null:
@@ -263,7 +281,9 @@ class RecordingEngine {
 
   /// Called for every GPS fix. **Must not block and must not touch the database.**
   void _onFix(LocationFix fix) {
-    if (!isUsableFix(fix.accuracyM)) return;
+    if (!isUsableFix(fix.accuracyM, maxAccuracyMeters: _activeProfile.accuracyGateM)) {
+      return;
+    }
 
     // Stamped at creation, which is what makes a pause safe: a fix already in flight
     // lands in the segment it was recorded during, not the next one.
@@ -277,7 +297,10 @@ class RecordingEngine {
       timestamp: fix.timestamp,
       latitude: fix.latitude,
       longitude: fix.longitude,
-      speedKmh: sanitizeSpeedKmh(msToKmh(fix.speedMps)),
+      speedKmh: sanitizeSpeedKmh(
+        msToKmh(fix.speedMps),
+        floorKmh: _activeProfile.noiseFloorKmh,
+      ),
       altitudeM: fix.altitudeM,
       accuracyM: fix.accuracyM,
       bearingDeg: fix.bearingDeg,