docs(ADR-0131.5): retire GSM8K probe as per-iteration gate after G.x completion (#199)

All five G.x capability axes shipped (G.1–G.5); admission remained 0/50
throughout — expected, because the 50 sample cases fail at the statement
layer (rate verbs, implicit entities, multi-step ops), orthogonal to the
question-layer and initial-state-verb work the G.x axes targeted.

Decisions:
- Probe retired as a per-iteration gate (signal-to-noise ≈ 0 at 0/50)
- `admitted_wrong == 0` invariant preserved as a standing architectural
  invariant, CI-pinned by existing test + per-axis regression guards
- Re-activation criterion named: statement-layer expansion that
  plausibly admits ≥1 new case (Δadmission_rate ≥ 0.02)
- Probe runner + report stay in-repo for future statement-layer push

Docs-only. No test, parser, solver, or eval changes.

docs/decisions/ADR-0131.5-gsm8k-probe-retirement.md

@@ -0,0 +1,124 @@
+# ADR-0131.5 — GSM8K Coverage Probe: Retirement After G.x Axis Completion
+
+**Status:** Accepted  
+**Date:** 2026-05-23  
+**Parent:** [ADR-0131.G — GSM8K Coverage Probe](ADR-0131.G-gsm8k-coverage-probe.md)  
+**Amends:** ADR-0131.G iteration discipline section
+
+---
+
+## Context
+
+ADR-0131.G introduced the GSM8K coverage probe as a diff-able baseline
+(`admitted_wrong == 0`, `admission_rate` tracked across G.x iterations)
+and pinned capability-first iteration discipline for the G.x capability
+axes.  Five axes shipped and merged to `main`:
+
+| Axis | ADR | PR | Axis cases | Wrong |
+|------|-----|----|-----------|-------|
+| G.1 — verb classes for initial-state | ADR-0131.G.1 | #187 | curated lane | 0 |
+| G.2 — comparative operations | ADR-0131.G.2 | #196 | 31 cases | 0 |
+| G.3 — numeric literals | ADR-0131.G.3 | #183 | curated lane | 0 |
+| G.3.1 — numerics extensions | ADR-0131.G.3.1 | #186 | curated lane | 0 |
+| G.4 — multi-clause composition | ADR-0131.G.4 | #185 | curated lane | 0 |
+| G.5 — aggregate answer composition | ADR-0131.G.5 | #197 | 20 cases | 0 |
+
+**GSM8K admission after all five axes: 0/50 (unchanged from baseline).**
+
+This was expected and documented in each G.x PR.  The bottleneck for all
+50 sample cases is statement-layer parsing — rate verbs, compound
+sentences, implicit entities, multi-step operations.  The G.x axes
+targeted the *question layer* (aggregate cues, comparative structure,
+numeric literals, multi-clause subjects) and the *initial-state verb
+layer*.  None of the 50 GSM8K sample cases fail at those layers; they
+fail earlier.
+
+The safety rail (`admitted_wrong == 0`) never wavered.
+
+---
+
+## Decision
+
+### 1. Retire the GSM8K probe from the per-iteration gate
+
+Effective immediately, `train_sample_coverage_report.json` is a **frozen
+evidence artifact**, not a live gate for future iterations.  The reasons:
+
+- Admission is still 0/50.  A gate that never moves does not gate.
+- The G.x axes demonstrated that architecture-aligned capability growth
+  does not necessarily lift GSM8K admission — and that is correct
+  behavior, not a failure.  The 50 sample cases are hard precisely
+  because of statement-layer features (rate verbs, implicit entities)
+  that are deliberately out of scope for the closed bounded-grammar
+  model.
+- Keeping the probe as a per-iteration required re-run creates process
+  overhead whose signal-to-noise ratio is now close to zero.
+
+### 2. Preserve the `admitted_wrong == 0` invariant permanently
+
+The safety rail is not retired.  It becomes a **standing architectural
+invariant** rather than a per-iteration gate:
+
+> Any future change to the parser, solver, or verifier must preserve
+> `admitted_wrong == 0` on the GSM8K train sample.
+
+The existing test
+`tests/test_adr_0131_G_gsm8k_coverage_probe.py` already CI-pins this;
+it is not removed.  Each per-axis test file (G.2, G.5, etc.) also
+asserts `admitted_wrong == 0` as a regression guard.  This coverage is
+sufficient; no per-iteration re-report is needed.
+
+### 3. Conditions for re-activating the probe as a live gate
+
+If a future ADR targets **statement-layer** expansion (rate verbs,
+multi-step implicit operations, compound sentence parsing), re-run the
+probe and include the delta.  The probe infrastructure remains in-repo
+for this purpose.  The re-activation criterion is:
+
+- The new capability plausibly changes the admission count on ≥1
+  of the 50 sample cases.
+- The PR diff includes evidence that it does (pre/post `admission_rate`
+  delta ≥ 0.02, i.e. ≥1 additional admission).
+
+### 4. Archive note in ADR-0131.G
+
+ADR-0131.G iteration discipline section 4 ("Reject any expansion that
+only moves GSM8K admission") remains valid.  This ADR adds a symmetric
+note: expansions that do not move GSM8K admission are not defects — they
+are working as intended when the bottleneck is orthogonal to the
+capability axis being extended.
+
+---
+
+## Evidence
+
+- `evals/gsm8k_math/train_sample/v1/train_sample_coverage_report.json`
+  — frozen at `admission_rate: 0.0`, `admitted_wrong: 0`, `refused: 50`
+  after all G.x axes.
+- `tests/test_adr_0131_G_gsm8k_coverage_probe.py` — CI-pins
+  `admitted_wrong == 0` as a standing regression guard.
+- Per-axis tests (G.2, G.5) each include a `test_gsm8k_*_admitted_wrong_zero`
+  assertion as belt-and-suspenders.
+
+---
+
+## Consequences
+
+- Future capability-axis PRs do not need to re-run or delta the GSM8K
+  probe unless their scope touches statement-layer parsing.
+- The probe file and runner remain in-repo as infrastructure for the
+  eventual statement-layer push.
+- The ADR-0131.G "capability-first, not coverage-first" discipline is
+  unchanged.  GSM8K admission is still a *symptom*, never a target.
+  This ADR merely removes the requirement to measure the symptom on
+  every iteration when the symptom cannot move.
+
+---
+
+## Out of scope
+
+- Statement-layer expansion (rate verbs, multi-step implicit operations)
+  — that is a separate ADR when the time comes.
+- Promotion-gate amendment — ADR-0131.4 is the gate; it does not
+  reference the GSM8K probe, and this ADR does not change it.
+- Removal of the probe runner or report file — both stay in-repo.