fix(transit): keep parked vehicles pointed along their course while the camera orbits (#603)

## Changes

Follow-up to #596 from the owner's look: a parked transit vehicle turned with the camera. Moving vehicles project their world course into screen space on every camera change, but a vehicle that had never moved on screen could have no display course and was skipped by that pass, leaving its billboard at a screen-anchored angle; rotation invalidation also relied on camera events that a pose change without a revision could miss. Now every visible vehicle with any usable course (derived, held, or the feed's reported bearing) is re-projected at the existing 5 Hz cadence on camera pose changes. Moving playback and the 120°/s course limiter are unchanged.

## How it was verified

- `npm run format:check` (861 files), `npm run check:boundaries` (109 groups), `npm test` (4,068 passing), `npm run build` clean; tracking regression 109/109 in a real browser on Google 3D tiles.
- Node tests: a stationary vehicle with a held course, and one with only a reported bearing, under a camera at heading 0° then 90° yield screen rotations 90° apart.
- Browser harness (`scripts/qa-transit.mjs`, keyed on Google 3D tiles): a parked scripted vehicle orbited by 90° rotates 90.000° on screen; a moving vehicle's rotation still matches its rendered travel; both checks fail on main and pass here. Full keyed run: 201 pass; the nine failures are the per-frame allocation gate already recorded as a follow-up and threshold-edge readability lines on the Boston matrix, both unchanged from #596.

scripts/format-scope.json

@@ -702,6 +702,7 @@
   "src/layers/transit/viewport.js",
   "src/layers/transit/lifecycle.test.mjs",
   "scripts/qa-transit.mjs",
+  "scripts/qa-transit-heading.mjs",
   "src/tooling/transitProvider.test.mjs",
   "src/app/layers/transit.js",
   "src/data/transitIcons.js",