fix(install): serialize cusum threshold crossing via cas

[imjustprism]

Fixed

• RegimeDetector::record race where two threads near the threshold both fire Rising for one regime change

Why

old path:

let pos = self.pos.fetch_add(dev, Relaxed).saturating_add(dev);
if pos >= threshold {
    self.pos.store(0, Relaxed);
    return Rising;
}

thread A and thread B both observe their post-add pos >= threshold (B's add happens after A's, so B sees an even larger sum). both reset to 0. both return Rising. limiter applies 0.7x shrink twice (~0.49x) for one regime change. floor bounds damage but throughput recovery slows.

Changed

CAS-guarded threshold crossing. only the thread that successfully transitions the over-threshold value to 0 returns Rising. racers see 0 on retry, treat their delta as a fresh accumulation against the cleared sum.

mirror logic on the Falling (neg) path.

Verified

check	result
cargo fmt --check	clean
cargo clippy --all-targets -D warnings	clean
cargo test (1453 tests across all crates)	pass
install --diag full on 132-direct / 2839-pkg fixture	clean, 69s wall
install on vue fixture (306 pkgs)	clean, 18.9s
install on next fixture	clean, 8.9s

bats not run (windows host lacks flock). CI bats covers it.

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[greptile-apps]

Greptile Summary

This PR replaces the fetch_add + conditional store(0) pattern in RegimeDetector::record with a CAS loop so that only one thread can claim a threshold crossing per event. Both the pos (Rising) and neg (Falling) paths are updated symmetrically.

• Race fix: the CAS ensures only the one thread that transitions the over-threshold value to 0 returns a signal; all concurrent losers receive Stable.
• Single-sample edge case: the Err(observed) if observed < current guard prevents a loser from re-entering the threshold arm after the winner has already cleared the accumulator — correctly handling samples whose individual deviation alone meets the threshold. The mirror guard on the neg path is identical in structure.
• Loser accumulation: losing threads do a fetch_add(abs_dev) onto the freshly-cleared accumulator so their deviation is counted toward the next genuine threshold crossing rather than being silently dropped.

Confidence Score: 5/5

The change is a well-scoped concurrency fix with no new data paths, no schema changes, and symmetric treatment of both accumulator arms.

The CAS loop correctly serializes threshold crossings, the observed < current guard closes the single-sample edge case from the previous review thread, and the loser fetch_add preserves deviation contributions for future accumulation. No correctness issues remain.

No files require special attention.

Important Files Changed

Filename	Overview
crates/aube-util/src/adaptive.rs	Replaces racy fetch_add+conditional-store with a CAS loop in both the pos (Rising) and neg (Falling) paths; the observed < current guard correctly suppresses losers even when a single sample's deviation alone meets the threshold, which addresses the edge case the previous review flagged.

_{Reviews (2): Last reviewed commit: "fixes" | Re-trigger Greptile}