fix(research-loop-cap): claim each turn slot with O_EXCL so the bound survives concurrency

countTurns-then-appendFileSync is read-then-write. N callers that all
observe used == budget-1 all decide to grant, and the bound is exceeded
by N-1. The comment above allowTurn asserted "Append-only: never
read-modify-write" and named the concurrent case - Phase 4.5/5 may spawn
several agents in a single message - as the reason it had to be. The
decision path was exactly what the comment denied, so the concurrency
claim had nothing under it.

Each grant now creates <data root>/trekresearch-loop-claims/<runId>-<slot>.claim
with flag 'wx' (O_CREAT|O_EXCL) before appending. The kernel picks the
winner per slot, slot numbers are bounded by the budget, and each can be
created exactly once - so total grants for a run cannot exceed the budget
however many callers arrive together. The ledger count now only says
where to start looking for a free slot.

Two of the three tests are deterministic and do not race anything: they
assert the invariant directly by pre-creating claims, including the state
a mid-append competitor leaves behind (ledger 7, slots 1-8 claimed, budget
8 -> deny). That matters because the third test - six real concurrent shim
processes at the boundary - passed even BEFORE the fix, since process
startup jitter serialised them. A race test that passes by luck is not
evidence, so it ships as a real-world regression guard next to the two
that are.

Stated rather than left to be discovered: claim files are empty, at most
budget per run, and never cleaned - the same standing as the ledger, which
also grows for the life of the data root. Reusing a runId across runs, or
two runIds colliding after filename sanitisation, both deny a turn, which
is the safe direction for a budget control.

Review finding 3994491ef1fdba6e0e3645b5b713cbdbdeb2b328 (MINOR).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LuGhWAbWyRFBFeemfhxoVv

docs/architecture.md

@@ -12,7 +12,7 @@ Imported from `CLAUDE.md` via pointer.
 - `lib/stats/event-emit.mjs` — single-source stats event emitter for autonomy-gate transitions and main-merge-gate (v3.4.0)
 - `lib/validators/{brief,research,plan,progress,session-state}-validator.mjs` — schema validators with CLI shims (`node lib/validators/X.mjs --json <path>`)
 - `lib/validators/architecture-discovery.mjs` — drift-WARN external-contract discovery for `architecture/overview.md`
-- `lib/util/research-loop-cap.mjs` — stateful, **default-off** turn budget for the `/trekresearch` bounded conversation loop. `allowTurn()` derives the used-turn count from its own append-only JSONL ledger; it never asks the caller how many turns it has spent, because a cap that does is not a cap. Budget = `TREKRESEARCH_MAX_CONV_TURNS` (default `3`, invalid values fall back to `3`) × `maxDimensions` (8, `settings.json:16`). Grants 0 unless `VOYAGE_STORM_ENABLED=1`, which is also the second condition on Phase 4.5's skip-guard (that phase does not call this module): unset, **both** STORM phases are inert. `resolveDataRoot()` is the single root for everything the loop writes — `CLAUDE_PLUGIN_DATA` when the harness sets it, `~/.claude/voyage` when it does not (it is empty in the Bash tool's process env, which is where the loop actually runs); the cap hook resolves through the same function, so writer and reader cannot disagree. A ledger that cannot be **written** denies the turn, and one that exists but cannot be **read** denies it too — only `ENOENT` counts as zero turns spent, that being the legitimate first-turn state (fail-closed — the opposite of `event-emit.mjs`, which is telemetry and must never block). The exported `readLedger()` is the single counting rule; the cap hook calls it rather than keeping a private copy. CLI shim: `node lib/util/research-loop-cap.mjs --run-id ID --dimension D --effort E`
+- `lib/util/research-loop-cap.mjs` — stateful, **default-off** turn budget for the `/trekresearch` bounded conversation loop. `allowTurn()` derives the used-turn count from its own append-only JSONL ledger; it never asks the caller how many turns it has spent, because a cap that does is not a cap. Each grant first claims a turn **slot** with `O_EXCL` under `trekresearch-loop-claims/`, so the bound survives several callers deciding at once — counting the ledger and then appending is read-then-write, and Phase 4.5/5 can spawn several agents in one message. Budget = `TREKRESEARCH_MAX_CONV_TURNS` (default `3`, invalid values fall back to `3`) × `maxDimensions` (8, `settings.json:16`). Grants 0 unless `VOYAGE_STORM_ENABLED=1`, which is also the second condition on Phase 4.5's skip-guard (that phase does not call this module): unset, **both** STORM phases are inert. `resolveDataRoot()` is the single root for everything the loop writes — `CLAUDE_PLUGIN_DATA` when the harness sets it, `~/.claude/voyage` when it does not (it is empty in the Bash tool's process env, which is where the loop actually runs); the cap hook resolves through the same function, so writer and reader cannot disagree. A ledger that cannot be **written** denies the turn, and one that exists but cannot be **read** denies it too — only `ENOENT` counts as zero turns spent, that being the legitimate first-turn state (fail-closed — the opposite of `event-emit.mjs`, which is telemetry and must never block). The exported `readLedger()` is the single counting rule; the cap hook calls it rather than keeping a private copy. CLI shim: `node lib/util/research-loop-cap.mjs --run-id ID --dimension D --effort E`
 - `lib/validators/query-privacy-gate.mjs` — gates **every** outbound research query before it leaves the machine; the hard-block tier (secret-shaped strings) is not operator-overridable, so a query that trips it must be reformulated rather than forced through. CLI shim: `node lib/validators/query-privacy-gate.mjs "<query>"`
 
 Wiring points (replaces previous prose-grep instructions):