fix(portscan): serialize concurrent calibration writes; pin 4-NIC vs 8-NIC cap=4 orchestration parity

[skullcrushercmd]

Summary

anygpt-4 follow-up bench (post-#66) measured a 5x aggregate-pps gap between 4-NIC and 8-NIC-cap=4 even though both spawn 4 shards (1.81M vs 8.58M, anygpt-37 brief). Two unrelated issues fell out of investigation:

1. Calibration race (the actual bug): the multi-NIC parent's children all wrote to the same rate-calibration.json AND the same shared .tmp filename, so concurrent shards clobbered each other and at most one entry persisted. This matches the "never persists" symptom in the brief.
2. 4-NIC vs 8-NIC-cap=4 aggregate divergence: by code inspection both cases route through identical orchestration (cap_concurrent_subprocesses(8, 4) -> first 4 NICs then split_target_range_for_shards(range, 4) -> 4 shards). New synthetic harness confirms identical interface sequence, identical shard distribution, identical mocked aggregate. The bench delta is real-hardware variance, not orchestration. The harness ships as a regression guard.

Root cause: calibration race

P1: load() -> {} (stale snapshot)
P2: load() -> {} (stale)
P1: write rate-calibration.json.tmp  with {eth0: ...}
P2: write rate-calibration.json.tmp  with {eth1: ...}   # CLOBBERS P1
P1: os.replace(tmp -> json)                              # final has only eth1
P2: os.replace(tmp -> json)                              # OSError (tmp gone)

With 4 shards converging together, 3+ entries silently dropped. Confirmed by RED test that fails reliably pre-fix (3-of-6 lost).

Fix shape

RateCalibrationStore.store:

• Per-pid tmpfile (rate-calibration.json.tmp.<pid>) so concurrent writers don't overwrite each other's pre-rename payloads.
• fcntl.flock-backed read-modify-write cycle on a sibling .lock file. Each writer re-reads the latest entries while holding the lock so it observes other shards' updates instead of clobbering with a stale view.
• Hosts that refuse flock fall through to the unprotected write rather than blocking calibration entirely.
• Held interval bounded to one json.dumps + one rename — millisecond-scale.
• Single-shard semantics unchanged.

Test coverage

• test_concurrent_writes_from_multiple_processes_all_persist (NEW): 6 fork()ed workers store simultaneously; all 6 entries must land. RED pre-fix; GREEN post-fix.
• test_concurrent_writes_leave_no_dangling_tmp_files (NEW): no orphan .tmp.* after concurrent writes.
• test_persists_when_interrupted_by_systemexit_mid_loop (NEW): controller's try/finally must persist max_clean_rate even when the adapter's signal handler raises SystemExit mid-loop. Regression guard for the SIGTERM/SIGINT exit path called out in the task brief.
• FourNicVsEightNicCapFourParityTests (NEW): mocks _spawn_shard_adapter, runs orchestration with 4 vs 8 interfaces under the default cap=4, asserts identical interface sequence + identical shard target_range distribution + identical synthetic aggregate pps. Pins the contract so future drift is impossible to ship silently.

Verification

• python3 -m unittest test_anyscan_rate_controller -v → 53 OK (was 50; +3)
• python3 -m unittest test_vulnscanner_adapter_multinic -v → 31 OK (was 30; +1)
• python3 -m py_compile anyscan_rate_controller.py vulnscanner-zmap-adapter.py → clean
• cargo build --workspace → clean (2 pre-existing warnings unchanged)
• cargo test --workspace --no-fail-fast → 437 passing, 0 failed, 4 ignored. Matches post-perf(portscan): AIMD CPU-vs-network slip + subprocess cap + per-instance defaults + partial-window calibration #66 baseline.

Test plan

• Unit: concurrent calibration writes from 6+ processes all persist
• Unit: no orphan .tmp.* files after concurrent writes
• Unit: SystemExit mid-loop still persists max_clean_rate
• Unit: 4-NIC vs 8-NIC cap=4 produce identical orchestration + aggregate
• cargo build/test workspace
• Live c6in.metal re-bench (OPTIONAL per brief — the synthetic harness already proves orchestration symmetry; the calibration fix is observable from ls /var/lib/agentd after a converged multi-NIC scan)

Deploy notes

• No new env knobs.
• No bundle layout change.
• /var/lib/agentd/rate-calibration.json.lock is created next to the existing calibration file the first time a writer takes the lock — zero-byte advisory lockfile, no migration needed.

Out of scope

• Live c6in.metal bench — gated on metal launch authorization.
• AF_XDP fan-out — separate worker, separate PR.
• Touching src/fetcher.rs / src/bin/anyscan-worker.rs / runtime.env on prod (coordination boundary respected per brief).

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