A3: record per-phase headroom diagnosis for docsync, plan-mode-no-op, mode-routing and gate slice 11

docs/evals/headroom-diagnosis-2026-10.md

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+# Headroom diagnosis, October 2026 (A1-A3)
+
+Per-phase timings for the cases the reliability plan named as slow, measured from
+stored CI artifacts of census 37198445662 (head `fd6854b`) and, for the one red,
+37176837432. Native captures carry message timestamps; PTY cases carry the
+assistant-message timestamps their observation files keep. Durations in the
+trial history (`wave-trials`) for multi-test files are the JUnit **sum** of tests
+that run concurrently, not wall time or session time, so they overstate headroom
+for `plan-mode-no-op` and `plan-ceo-mode-routing`.
+
+## A1. ship-docsync-completion and -store (session budget 585 s)
+
+History: completion 349-517 s, store 377-466 s across the eight wave censuses.
+
+| Phase (37198445662) | completion | store |
+|---|---|---|
+| Prepare: read phase, skills, Git state, fixture files; snapshot | 85 s (two thinking gaps of 9 s and 26 s) | 99 s (33 s thinking) |
+| Compose child prompt (9.5 KB / 7.1 KB `Agent` input) | 43 s | 32 s |
+| Child (`document-release`, foreground) | 122 s | 118 s |
+| Validate (compare, Git reads, small file reads) | 3 s tool time, 38 s thinking | 3 s tool time, 13 s thinking |
+| Write records after the child | 45 s (section copy, report, result JSON with typed hashes) | 78 s (child output copy, section, record, report) |
+| Publish stand-in, verify, final message | 32 s | 25 s |
+
+Tool time in the parent is about 1 s per session. Reading release-file contents
+does not dominate (10 small files, ~1 s), so the parent keeps its reads and the
+child result contract stays unchanged. The parent's own model time dominates
+(~270 s of ~390 s). Two parts are cuttable without weakening the gate:
+
+- **Records after the child** (product): the parent retyped hash maps and child
+  output into several files. The gate now records post-child hashes by rerunning
+  the `gstack-docs-candidate snapshot` command (`--out <audit-id>-post.json`),
+  Step 16 runs `compare` on it, and each record is saved once and cited by path.
+- **Child prompt copy** (fixture only): ~70% of the child prompt is the fixture's
+  observation interface, which the fixture tells the parent to copy into child
+  prompts (`docsNativeInterface`, "include this interface in child prompts").
+  Writing that interface to a fixture file the child reads would save ~30-40 s per
+  session; it touches the shared detector/actor module, so it is a lane D request.
+
+## A2. plan-mode-no-op and plan-ceo-mode-routing
+
+**plan-mode-no-op is not a headroom problem.** Its six PTY sessions start
+concurrently; 37198445662's file wall is 263 s (slice entry 271.6 s) while the
+JUnit sum is 518 s. The slowest session is 159 s of its 300 s observation budget
+(53%). It runs in gate slice 4, not the 711 s critical-path slice 11.
+
+**plan-ceo-mode-routing** runs two concurrent sessions (wall 378 s, sum 687 s).
+The slowest session (HOLD SCOPE) is 378 s of 600 s (63%). The tight budget is the
+test's 240 s post-selection window:
+
+| HOLD SCOPE session (37198445662) | elapsed |
+|---|---|
+| Preamble | 17 s |
+| Pre-review audit batch | 46 s |
+| Setup questions and answers | 45 s |
+| Learnings search, Step 0 analysis written to the plan | 50 s |
+| Mode question rendered and answered | 20 s |
+| Handoff helper, plan write, two section reads, Codex probe, then posture chat | 114 s |
+| HOLD continuation question and posture assessment | 76 s |
+
+In the red (37176837432) the session skipped the handoff helper and paraphrased
+the mode ("holding scope"), then spent ~60 s per decision on the ledger
+save/read-back/ask cycle until the 240 s window closed. The skill now sends the
+handoff line before any other tool call, plan write or section load, and never
+skips the helper. The remaining time is model latency of the decision procedure
+(about 60 s per question: comparison grid, save, Read-back, ask); cutting that
+would weaken the question-persistence gate, so it stays a named item.
+
+## A3. Census critical path
+
+Gate slice 11 (711 s, the census critical path) holds 17 short files. Their
+recorded estimates (`scripts/paid-test-durations.json`, recorded 2026-09-29)
+total 1078 s; they ran 1353 s. Four files ran about twice their estimate in
+every wave census:
+
+| File | estimate | wave census range |
+|---|---|---|
+| `skill-e2e-plan.test.ts#plan-review-report` | 76 s | 168-273 s |
+| `skill-e2e-plan-ceo-plan-mode.test.ts` | 37 s | 85-120 s |
+| `skill-e2e-deploy.test.ts#benchmark-workflow` | 62 s | 59-121 s |
+| `skill-e2e-safety-codex-consult.test.ts` | 83 s | 111-212 s |
+
+None is near its own budget (plan-review-report peaks at 46% of 600 s). The fix
+is a refreshed duration seed so the planner packs slice 11 within the 540 s slice
+budget (lane F / integrator). review-army-perf and plan-design-review-plan-mode
+need no work.