Floor memory heartbeat: make the reclaim-throttle wedge attributable from the log

[gunbai-bot]

Lever 2 of the 2026-07-11 memory-mechanism discussion (operator: "lets do 2" — keep it simple and effective). Fidelity-only: reads state, changes no behavior.

Why: the throttle incident's signature — four floors on three hosts with memory.peak pinned at memory.high + <1MiB through 30–49-minute log-silent tails — was invisible in the log because memory.high throttles instead of killing: the kernel swaps the floor to a crawl with zero evidence, and the only receipt was a post-hoc peak read.

What: one synchronous regime-disclosure line at floor start (which memory.high/memory.max bind, and where), then one line per minute from a detached thread: memory.current, memory.swap.current, memory.events high-throttle count, PSI some avg10. The wedge signature becomes current pinned at high, swap climbing, high-events exploding, PSI double-digits — attributable to a 60-second window. Sampling denominator = binding-high dir when set (the slot slice that throttles), else binding-cap, else leaf; absence of all three refuses loudly and the floor proceeds unmonitored — never a fabricated zero.

Shape: binding_cap_cgroup_dir's leaf→root walk factored into tightest_cgroup_dir_for (one walk, two limit files — no copied sibling); parsers pure over file content with discriminating unit tests (exact-key match proven against the oom/oom_kill prefix trap).

Receipts: 3/3 unit tests green; by-execution smoke on the real composed floor (this box, 31.3GiB memory.max regime):

[floor-memory] regime: memory.high=none; memory.max=33578549248 (/sys/fs/cgroup/)
[floor-memory] t=1m current=4265451520 swap=0 high_events=0 psi_some_avg10=0.00
[floor-memory] t=2m current=5804589056 swap=0 high_events=0 psi_some_avg10=0.00

~60 lines/hour worst case against a floor log that prints thousands of SKIP lines.

🤖 Generated with Claude Code

[gunbai-bot]

The failing ci check is not this diff: batch 3 fails generated_artifact_drift_gate_passes, which is red on main (runs 29165555785 / 29167030699 — Lane D #6474 hand-added cross-links to two generated plan docs without backing .dag rows). This PR's merge-ref inherits it; the fix is #6503. Once #6503 merges, a re-run here goes green — the heartbeat diff itself is Rust-only telemetry with 2/2 approvals. — sent from eager-gull-872