feat(transit): live public-transit vehicles from open GTFS-Realtime feeds (#587)

Supersedes #218 by @spcpza, ported onto the current component layout with fixes from review.

Verified before merge: format, package boundaries, unit suite, build, and the browser tracking suite; a browser harness for the change where one applies; adversarial review applied.

Contributor: @spcpza. His original commit is the base of this branch, and his own re-port onto main (2026-09-15, `5b69011`) was consulted while finishing the height work. That port reads every vehicle's surface from the app's shared ground-floor system (`cachedGroundFloor` / `warmGroundFloor` / `GROUND_FLOOR_LIFT_M`) rather than sampling the scene per layer, keeps a vehicle hidden until its cell answers, warms a bounded batch per poll on a timer instead of per frame, and renders at the ellipsoid above the altitude gate. That is the sounder primitive — one floor system for the app, with the rooftop-versus-street question already settled against a datum-correct DEM prior — and this branch's height work was rebuilt on it, replacing a bespoke cell cache, ground prior and rooftop rule written here. Credit for that direction is his.

Future work (not in this PR), both written up in docs/CURRENT-STATE.md:

- Readable route names. The realtime feeds carry only `route_id`; `route_short_name` / `route_long_name` live in each operator's static GTFS `routes.txt`, and headsigns in `trips.txt`. Doing it properly wants a server-side metadata loader beside the transit proxy with fixed registered static sources, bounded indexes built once and persisted, a slow conditional refresh, and a versioned route dictionary the browser caches. Shipped here: ids are never rewritten, and a cut one carries an ellipsis instead of reading as a typo.
- Below-ground rendering. Subway vehicles are drawn on the surface directly above themselves, because the feeds carry no depth and a train nobody can see is not worth drawing; the card says so plainly. Real treatment wants OSM tunnel geometry per city and a ghosted sprite tethered to the surface. The owner recorded it and left it.

Round four and after (2026-09-15). The owner tested round three under FLIR and NVG, and then asked for the design to be taken further: elegant in performance, beautiful on camera, trails with a cache for public transport, Cesium-native, and a core other moving layers can adopt. Codex Astra reviewed the plan (it confirmed the speed fault: a glide started from the drawn position at receipt time but spanned the gap between report times, so with MBTA reporting every ~10 s against a 15 s poll every second glide ran at 1.5x; it rejected a queued-segment patch), then wrote the design, then implemented it in commit-sized passes with adversarial review between them and real-browser harness runs by the integrator.

What this PR now contains beyond the contributor's original port:

- Delayed playback in a contact-agnostic core (`src/data/contactPlayback.js`): each vehicle keeps a typed, bounded fix history and a display clock that runs at one second per second through it, a feed-derived lag behind real time, holding at its newest fix rather than extrapolating and never sprinting to catch up. Marker, course, motion word, card and trail all read the same displayed sample. The clock has an explicit time-offset and seek API so a rewind is a later feature, not a rewrite; flights and vessels are untouched.
- A selected-vehicle trail (`src/data/contactTrailRenderer.js`, `src/layers/transit/trails.js`): the retained history drawn with batched Cesium primitives, clipped at display time so the head is exactly the sprite, faded by age, in the mode colour (white under sensor presets), on the same surface path as the marker.
- Bounded proxy-side history (`server/providers/transitHistory.js`): fifteen minutes and at most 128 fixes per vehicle with hard byte ceilings, MBTA only for now (opt-in per feed), served by a small endpoint that never fetches upstream, so a selected trail survives a browser reload while the server keeps running.
- Scene work bounded to what is visible and moving, an occluder, cached DETECT candidates, holds on the render loop only while something visible moves.
- Sensor presets: white-hot haloed sprites under NVG/FLIR/noir (black-hot inverts), saturated mode colours under CRT, per-mode bracket tiers in every theme, the selected card keeping its mode colour.
- A discriminating browser harness (`scripts/qa-transit.mjs`) that samples postRender frames, reads the real card anchor, and measures fleet budgets at 800 and 3,000 vehicles.

Not built, recorded as follow-ups: curated route display names (lowest priority per the owner), fleet-wide tails, route snapping, a rewind UI, adoption of the playback core by flights and vessels.

Verified at merge (2026-09-15 morning): unit suite 3,896 passing, build clean, format and boundary gates clean, tracking regression 109/109, browser harness keyless 200 pass / 0 fail (13 sensor checks unexercised because the live view offered fewer than six isolated sprites; every judged sprite passed), keyed on Google 3D tiles 198 pass with the per-frame allocation gate as the only failure. Follow-ups recorded: the keyed build's playback sampler still allocates per frame (CPU and frame time stay within budget; the 3,000-vehicle metro view sits at a 27.6 ms frame interval against a 25 ms budget), the shared detection overlay's per-paint allocation, friendlier route names, fleet-wide tails, a rewind UI, and adopting the playback core for vessels and flights.
Co-authored-by: spcpza <[email protected]>

SECURITY.md

@@ -64,6 +64,7 @@ The data proxies under `server/providers/` are written so the browser cannot tur
 - **No arbitrary-URL fetching.** The CCTV frame proxy fetches only server-registered camera/frame URLs — clients cannot pass an upstream URL to fetch (SSRF mitigation). Other proxies target fixed upstream hosts.
 - **Radio is not an audio relay.** `/api/radio/stations` contacts only allowlisted Radio Browser HTTPS hosts and paths, rejects redirects, rejects any hostname with a loopback/private/link-local/metadata/non-public A or AAAA result, and pins each TLS connection to a validated address. It returns normalized public HTTPS stream URLs; `/api/radio/click/:uuid` applies the same destination policy and accepts only station IDs from the current bounded catalog. The browser then connects directly to the broadcaster after an explicit playback action, so the broadcaster sees the listener's IP address. GEV never proxies, caches, records, or redistributes audio.
 - **No verbatim client headers upstream.** The CCTV media route is the one route that relays a request header (`Range`, for video seeking). It is parsed and canonicalized before it is forwarded: one `bytes=` range only, with every accepted form — explicit span, open-ended and suffix — bounded to the same 64 MiB ceiling the relay applies to a declared response body. Multi-range, malformed, inverted and non-`bytes` values are dropped and the request proceeds without a `Range`, as RFC 7233 §3.1 prescribes. Bounding the request bounds what is asked for: a response that declares no length — live streamed media, or a chunked body from an upstream that ignores the `Range` — has no ceiling. An upstream request the browser has stopped waiting for is cancelled rather than left running, whether the viewer leaves before the headers arrive or during the body.
+- **Transit fetches registered feeds only.** `/api/transit/vehicles/<id>` resolves the id against `src/data/transitFeeds.js`; the browser never supplies a URL, and a feed that is registered but disabled does not resolve at all. Redirects are followed manually and each hop is validated against the feed's own https origin before it is requested, so an off-origin or downgraded hop is refused rather than contacted. Bodies are capped at 8 MB, the protobuf is decoded server-side under entity-count and string-length ceilings, a differential feed is refused, and snapshots live 15 s in memory with no disk cache. A per-feed admission limiter and a failure cooldown ladder bound what this process can ask of any operator.
 - **Response-size caps and timeouts** on proxied responses.
 - **Sanitized errors** — internal error details are not echoed back to clients.
 - **Coalesced OAuth refresh** and cached successful responses only (OpenSky).