fix(steer): the stall warning re-logged every tick while its own event path deduped

[Dygreens]

One undeliverable message to a lane dead six days produced 921 of the 1004 log lines since the last restart — 92% of the log — and 3570 across the preceding 24 hours. Same lane, same reason, same single queue row, re-stated every tick for 20 hours.

WARN steering queue STALLED — messages are not reaching this lane
     session=Amux-gtm queued=1 oldest_min=1180 reason=not-running

This is not a new policy — it's aligning two surfaces that report one fact

The emit_event calls immediately below this warn were already deduped on steer-stalled:{lane}:{cond}:{bucket} — hourly, per lane, per condition. Their comment names the exact hazard:

A permanent idem would fire once and stay silent if the lane recovered and re-stalled; no idem at all fires every tick, which is the nag AC-310 was filed about.

The event path was fixed. The tracing::warn! four lines above it was left behind.

So the log line now shares that key rather than getting a cadence of its own — two spellings of "the same stall" would drift, and the drift is the defect. blocked/cond/bucket move above the warn; same calls, same frequency, only the order changed.

In-process rather than DB-backed, deliberately: the event idem is durable because a missed notification is lost, whereas re-stating an ongoing problem once per server start is useful rather than noisy.

What it cost

It buried a first-ever database is locked line and two false scheduler warnings during a log review that existed to find exactly those. Ranking by severity instead of volume is the only reason they surfaced.

Test

Pins the two properties a careless fix breaks. Both verified to fail against a deliberately wrong implementation:

wrong implementation	caught by
always-log (the pre-fix behaviour)	assertion 1
permanent idem (the over-correction)	assertion 4

The second matters more. An idem that never rolls would fire once and then hide an ongoing stall forever — worse than the bug it replaces, and the surrounding code makes exactly this point.

It also asserts a different condition and a different lane still log, because collapsing those is what the surrounding code says made the stall invisible in the first place.

Not done here — a design call, not a patch

The queue row itself. A steer aimed at a lane dead six days should arguably be expired or escalated rather than retried forever. That's the better fix and it makes this one mostly unnecessary — but it decides someone else's pending message and belongs with the steer delivery path (LR-21). Recorded on the card rather than guessed at.

Verification — CI's exact commands

cargo check  --workspace --all-targets              clean
cargo clippy --workspace --all-targets -D warnings  exit 0
cargo test   --workspace                            39 result lines, 0 failed

🤖 Generated with Claude Code

https://claude.ai/code/session_01B4vuEScunv4K6RMwQoT9xx