Replace the hardcoded lane: 0 on input_received events with a real
device->lane lookup. A new LaneMap caches lane_devices.id -> lane,
built at startup and refreshed by the setup routes on assign/unassign.
An unmapped device logs lane: -1 + a warning (0 is a real lane) and is
still recorded faithfully (append-only chain).
source stays null for raw inputs by design: it's an IdentitySource
(how a vehicle was identified), not a device field; device provenance
remains in identity. Documented both in the wiki.
