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>

README.md

@@ -72,6 +72,8 @@ speed-derived values, where Kotlin's 32-bit `Float` widens with artefacts Dart's
 | Document | Contents |
 |---|---|
 | [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) | Why it is built this way, and what changed from the native app |
+| [docs/v3/](docs/v3/) | v3 tickets — one file per feature, ready to pick up |
+| [docs/BACKLOG.md](docs/BACKLOG.md) | The v3/v4 reasoning behind those tickets |
 | [docs/LAUNCH.md](docs/LAUNCH.md) | Getting from "on my phone" to the app stores, in stages, with costs |
 | [docs/port/IOS-VERIFICATION.md](docs/port/IOS-VERIFICATION.md) | What a Mac can prove about the iPhone build, and what cannot |
 | [docs/port/PLAN.md](docs/port/PLAN.md) | The 28-task migration plan |