[PurpleSentinel]

Closes #214.

The defect

TRIGGER = GPS armed the device, showed WAITING FOR THE LINE, and nothing could ever start the session. Only two call sites reached begin_session: a manual START and IMU launch detection.

The mechanism existed and was never called. session/gate_automation.cpp starts on pit exit and stops on pit entry, gated on settings session_trigger.cpp:11 already derives from the trigger, and it is host-tested. The crossings were already arriving — drain_timing_output() read them off the timing queue and discarded them in an empty while loop.

Two things beyond the missing call

The duration. SessionController::start takes its duration from the configuration already saved, and that was only ever written inside begin_session. A crossing-started session would have run for SessionConfiguration's own twenty-minute default rather than the driver's setting — plausible, and wrong for anyone who set something else. Configuration now follows the settings: written at boot and whenever a duration is edited, but not mid-session, since a running session keeps the duration it began with.

I initially wrote this up as a zero-length session, which was wrong — the struct defaults to 20 minutes. My own test caught it when == 0 failed; the issue carries the correction.

The completion reason. The summary hardcoded driver_stop. Harmless while that was the only way a session could end; a pit entry can end one now, and a record saying the driver stopped when they did not is worse than no record of how it ended. It is taken from the controller, which already sets CompletionReason::pit_entry.

Decisions worth flagging

Crossings are offered to the automation unconditionally, not only while armed on GPS. The two settings it consults are the authority: pit-exit auto-start is derived from the trigger, and pit-entry auto-stop is its own setting a driver can want with any trigger. Gating in the router as well would put one decision in two places.

stop_from_pit_entry goes straight to review having already refreshed the session times, so the summary is captured after it rather than before as the driver-stop path does, and confirm_stop is not part of this route.

Every outcome is logged except the start and finish gates, which come through the same queue several times a lap. A pit exit that did not start a session is otherwise indistinguishable from no pit exit, and the reasons differ: the setting is off, a session was already running, or the controller refused the transition.

Scope: lap timing is untouched

Lap timing runs in the timing task, consumes the fix queue and detects start/finish crossings itself. gate_automation only ever looks at pit_exit and pit_entry. Laps were always timed correctly with GPS selected — only the session clock never started.

Verification

• make check → CHECK_RC=0
• firmware builds clean, 0xfdf00, 32% free
• make repo-check passes
• New host tests: an unconfigured controller taking the struct default, a configured one honouring 35 minutes rather than it, a second pit exit not restarting a running session, and a pit entry ending one with its own completion reason

Confirming it against a real pit exit needs a circuit, so device-side acceptance waits on #204's gate capture. The arming, configuration and logging paths are all bench-provable.

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