feat(run-journal): measure lifecycle journal ceiling for #635

Extend the measurement harness with volume mode (real journal scan,
representative and worst-case synthetic runs) and record the raise-bounds
design decision before any production ceiling change.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: solomonneas

docs/phase-635-journal-ceiling-decision.md

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+# Issue #635: journal ceiling decision (measure, then design)
+
+Status: design decision awaiting grading. No production implementation in this change.
+
+## Measured numbers
+
+Artifact: `.brigade/measurements/issue-635-journal-ceiling.json` (local, same shape family as `issue-568-slice6.json`: environment metadata plus structured aggregates; volume mode adds bounds and scenario blocks).
+
+Current hard bounds: `MAX_JOURNAL_EVENTS = 512`, `MAX_JOURNAL_BYTES = 8 MiB`.
+
+| Scenario | Events | Bytes | % event ceiling | % byte ceiling | Hit ceiling? |
+| --- | ---: | ---: | ---: | ---: | --- |
+| Scanned real runs (n=13, single-seat journal-hardening runs) | p50/max 17 | p50 14,279 / max 14,286 | 3.3% | 0.17% | no |
+| Synthetic representative (1 seat, 1 attempt, 0 pauses) | 11 | 7,780 | 2.1% | 0.09% | no |
+| Synthetic roster-sized (11 seats, 2 attempts, 3 pause/resume cycles) | 155 | 124,777 | 30.3% | 1.5% | no |
+| Synthetic worst case (32 seats, 3 attempts, 8 pause cycles) | 512 | 420,683 | 100% | 5.0% | yes, mid-dispatch |
+
+Worst-case stop reason: `LifecycleJournalError: bound exceeded: journal event sequence above MAX_JOURNAL_EVENTS` at `seat-28` attempt 1. Bytes were still ~5% of the 8 MiB cap.
+
+## Decision
+
+Raise the event bound. Do not segment the journal yet.
+
+Reasoning:
+
+1. Event count is the binding constraint. At a full 512-event journal the file is only ~0.4 MiB.
+2. Realistic and roster-sized load stays far from 512 (3% real, 30% for 11 seats with retries and three approval cycles).
+3. The ceiling is reachable, but only under a synthetic fleet larger than today's 11-seat roster with three attempts per seat.
+4. Segmentation (cross-linked `lifecycle.NNNNN.jsonl`, reader/projector/doctor/recovery awareness) is a large surface change. The measured gap does not justify that cost before a cheaper bound raise plus a soft warning.
+5. Keep segmentation as the explicit escape hatch if #593 budget events or larger fleets push roster-sized runs past ~50% of the raised ceiling.
+
+Recommended production change (separate implementation issue, after this design is graded):
+
+- Raise `MAX_JOURNAL_EVENTS` from 512 to 2048 (4x).
+- Leave `MAX_JOURNAL_BYTES` at 8 MiB for now (still ~20x headroom at 2048 median-sized events).
+- Add a doctor soft threshold at 75% of the event bound (1536 of 2048): `WARN`, not `FAIL`.
+- Keep the hard refuse-to-append behavior only at the new ceiling, and make that failure visible (already true today via `LifecycleJournalError`).
+
+## Soft-threshold doctor warning
+
+When `brigade doctor` inspects a run journal and `event_count >= ceil(0.75 * MAX_JOURNAL_EVENTS)` while still under the hard ceiling:
+
+- status: `WARN`
+- check name: `runs: journal event headroom`
+- detail shape: `lifecycle journal at {event_count}/{MAX_JOURNAL_EVENTS} events ({pct}%); raise or segment before the hard ceiling halts appends`
+
+Below 75%: silent on this check. At or above the hard ceiling: keep today's `FAIL` / `bound exceeded` path for recovery verdicts that already treat oversize journals as failed.
+
+## Recovery after upgrade for a run already at the old ceiling
+
+A run that stopped at sequence 512 under the old bound has a complete, chain-valid journal and paired checkpoints. It does not need rewrite or segmentation to become readable again.
+
+After the bound raise ships:
+
+1. Readers (`read_journal` / `read_journal_bounded`) accept the existing 512-event file because it is under the new ceiling.
+2. Doctor recovery checks that previously returned `FAIL` / `bound exceeded` solely for the old event ceiling clear once the constant moves.
+3. If the run is still live or being resumed, the active lock owner may append sequence 513+ again; checkpoint pairing and projection continue from the existing chain tip.
+4. No migration tool is required for journals that stopped exactly at 512. Journals that somehow exceed the new byte bound remain fail-closed (unchanged).
+
+## Explicit non-goals for the follow-up implementation issue
+
+- No journal segmentation.
+- No change to worker/provider event streams (#592 stays separate).
+- No silent truncation or dropping of lifecycle facts when the hard ceiling is hit.
+
+## Reproducing the measurement
+
+```bash
+python scripts/measure_run_journal.py \
+  --mode volume \
+  --scan /path/to/.brigade/runs \
+  --output .brigade/measurements/issue-635-journal-ceiling.json
+```