feat(realtime): real-time GTFS-RT departure delay buffer

[Multipixelone]

What

Adds C1 — real-time GTFS-RT departures: when the subway/LIRR/bus line a rider is about to board is actually running late, fold a capped, observed delay into the departure buffer so the alarm fires earlier on a degraded-service morning.

Conservative by design (the "honest, not misleading" call): a live delay can only move the leave time earlier — never later — and it never claims a precise new arrival (that would need static trip data we don't carry). It mirrors the existing weather-buffer seam end to end.

How it works

• realtime.py — realtime_delay(route, at_time, config) -> RealtimeDelay, with an injectable fetcher, a 60s per-system TTL feed memo, and a fail-open contract (any error/miss → zero buffer, never a failed plan). It takes the first boarding leg, fuzzy-matches its stop name to GTFS stop_ids (rapidfuzz over bundled tables, scoped by system), fetches only that system's trip-update feeds, picks the predicted departure closest to schedule, floors at zero, caps at max_buffer_minutes, and ignores sub-threshold jitter.
• gtfs_rt.py — shared feed fetch/validate helper, reused by mta.py (alerts) and realtime.py (trip updates).
• Bundled data — data/stops_{subway,lirr,bus}.csv (generated from MTA static GTFS by scripts/build_stops.py).
• Wiring — RealtimeConfig (disabled by default); Plan.realtime_buffer_minutes / realtime_reason; planner folds the delay into the buffer alongside weather; format.py surfaces a 🚇 +N min — Q running ~6 min late advisory.
• CLI + skill parity — commutecompass realtime diagnostic command, realtime.sh, SKILL.md row, allowlist + example-config docs.

Design decisions (settled up front)

Decision	Choice
Delay semantics	Conservative buffer + advisory (leave earlier only)
Stop mapping	Bundled stops.txt subset + rapidfuzz
Feed scope	Subway + LIRR + bus

Testing

• tests/test_realtime.py: delay/cap/threshold/window/fuzzy/system-scoping/LIRR/bus/fail-open + feed parsing.
• Planner + CLI coverage added.
• mypy strict: clean · ruff: clean · full suite 645 passing · coverage 84% (≥80% gate).

Packaging note

Couldn't run a real python -m build in the dev sandbox (no build frontend / offline). The CSVs are confirmed not gitignored (hatchling bundles them by default) and explicit artifacts/sdist entries were added as backup. Worth a quick python -m build && unzip -l dist/*.whl | grep stops before release.

Out of scope (follow-ups)

• Full real-time reschedule (pick a different train, recompute arrival) — needs static schedule data.
• Delay on downstream/transfer legs (only the first boarding leg is actionable here).
• GTFS-RT vehicle positions; LIRR branch/track-level precision.