Break v3 into 16 tickets

One file per feature, in the v2 shape that worked: goal, context, design,
implementation, acceptance criteria, tests, risks, out of scope -- written before
implementing so the risks are on paper rather than walked into.

Three carry more weight than their size suggests. V3-01 ships the port's first
real migration, and since the destructive fallback is gone, getting addColumn and
a v1-database test right matters more than the feature. V3-08 forces a
routing-engine decision with ongoing cost, so it sits behind a RoutingService
interface mirroring what LocationSource did for GPS. V3-13 is not code at all --
it answers the three questions open since v2, and V3-15 may close unbuilt as a
result, which is a legitimate outcome.

Several tickets record constraints that are easy to lose: no activity picker in
front of Start, because the founding premise is press-and-go with gloves; the
Route-to-Trip foreign key must not cascade, or deleting an old plan deletes the
ride; V3-14 will deliberately break the parity harness by adding GPX <type>, and
that expectation should be updated rather than the check dropped; and V3-16 must
not regress the explicit text colours that exist because of the black-on-black bug.

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

docs/v3/V3-03-units.md

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+# V3-03 — Distance and speed units
+
+**Phase** Foundations · **Depends on** V3-02 · **Size** S · **Status** Not started
+
+## Goal
+Imperial as well as metric, chosen once and applied everywhere.
+
+## Context
+Everything is hardcoded metric: `formatDistance` switches m/km at 1000, `formatSpeed`
+prints km/h, elevation prints metres. Fine in Canada, useless to anyone in the US or UK.
+
+Cheap, and the kind of thing that makes an app feel unfinished when missing.
+
+## Design
+A `UnitSystem` enum (`metric`, `imperial`) in `Config`, defaulting from the device locale
+on first launch.
+
+**Conversion belongs in formatting only.** Storage stays SI — metres, km/h, metres of
+altitude — forever. Converting at the storage layer would corrupt every existing ride and
+break the parity harness.
+
+| Value | Metric | Imperial |
+|---|---|---|
+| Distance | m / km | ft / mi |
+| Speed | km/h | mph |
+| Elevation | m | ft |
+
+## Implementation
+1. `UnitSystem` in `Config`; default from `Platform.localeName`
+2. Extend `ui/format.dart` — every formatter takes the unit system
+3. Thread it through: record screen, trips list, trip detail, charts, map legend
+4. Exports stay SI regardless. GPX is metres by specification; changing that breaks
+   consumers.
+
+## Acceptance criteria
+- [ ] Switching units updates every screen immediately
+- [ ] Stored values are unchanged — verified by exporting before and after
+- [ ] GPX/GeoJSON output is byte-identical across the two settings
+- [ ] First launch picks a sensible default from the locale
+
+## Tests
+- Formatter tests for both systems, including the m→km and ft→mi boundaries
+- **A test asserting export output does not change with the unit setting**
+- Widget test toggling units and checking a rendered label
+
+## Risks
+The obvious trap is converting too deep in the stack. Guard it with the export test.
+
+## Out of scope
+Temperature, pace (min/km) — pace is arguably right for running, revisit after V3-01.