docs(#1820): add CONTEXT.md Spec-Section Helper Module entry (R1 review)

Addresses trek-e's R1 blocker: src/spec-section.cts is a new named module
but had no ## Domain terms entry in CONTEXT.md (INVENTORY.md had one).
Adds the ### Spec-Section Helper Module paragraph covering the owned
surface (SpecSectionKey, SECTION_HEADERS, SectionStatus, countSectionDataRows,
specSectionStatus), the suffix-tolerant header matching invariant, the
supplied = present AND dataRows > 0 rule, and the source-of-truth note
(gitignored per ADR-457). Maintainer owns final wording.

CONTEXT.md

@@ -323,6 +323,9 @@ The ownership seam between the prohibition probe and security/compliance tooling
 ### Prohibition Probe Module
 Second adapter of the Probe Core Module (ADR-550 Decision 7): the spec-phase prohibition-completeness probe wired into spec-phase Step 5.6, surfacing the unwritten *must-NOT* constraints (values/safety/ethics) the spec never forbids. Unlike the Edge Probe, recall is **prose-orchestrated, not a compiled engine** (ADR-550 D7b) — a two-stage pass per requirement: Stage 1 an adversarial recall question, Stage 2 a one-pass precision classifier (drop routine engineering, keep genuine prohibitions). The code surface is schema/projection only: `projectProhibitions()` (deterministic SPEC↔`must_haves.prohibitions` projection backing the `DEFECT.GENERATIVE-FIX` parity assertion), the `{test, judgment}` `PROHIBITION_VALIDATORS`, `validateProhibitionResolution`, and `dispositionForProhibition()` (the fail-closed default — an unwired `test`-tier item resolves to `unverified`/flagged, never green). **Deterministic test-tier locate (#1278, ADR-550 D3 addendum):** a resolved `test`-tier prohibition MAY carry an optional flat-scalar `check` descriptor — `check_kind` (`node-test` | `lint-rule`), `check_target`, and `check_rule` (lint-rule only) — that `projectProhibitions` emits into `must_haves.prohibitions` when well-formed, and `descriptorFromProjection()` (the read-back seam in the #1259 enforcement producer, `src/prohibition-enforcement.cts`) reconstructs into a `{kind, target, rule?}` `CheckDescriptor`, so verify-phase locates the wired check with zero LLM/author authoring. **Flat scalars, never a nested `check:{}` object** — so the round-trip rides the *unchanged* shared `parseMustHavesBlock` (the #644 no-parser-rewrite precedent); an absent/partial descriptor falls through to the producer's existing fail-closed locate, and `failFirst` stays caller-attested (machine-proof is #1279). No `proposeProhibitions()` — recall is LLM prose. `plan-phase` lifts every resolved prohibition from the SPEC `## Prohibitions (must-NOT)` section into the `must_haves.prohibitions` sibling block (never `truths`). Exports (locked surface): `projectProhibitions`, `PROHIBITION_VALIDATORS` (the `{test, judgment}` validators bundle injected into `probe-core`'s generic engine), `validateProhibitionResolution`, and `dispositionForProhibition` (the fail-closed disposition) — the prohibition adapter surface, shipped from `probe-core` alongside the generic engine. Source of truth: `gsd-core/bin/lib/probe-core.cjs` (the prohibition exports live in `src/probe-core.cts`, gitignored per ADR-457) + `gsd-core/references/prohibition-probe.md`. Tests: `tests/prohibition-probe.*.test.cjs`. See ADR-550, Probe Core Module, Edge Probe Module, Verification Tier, Bespoke vs Canon Prohibition.
 
+### Spec-Section Helper Module
+The SINGLE source of truth for "did the phase SPEC supply section X (with at least one resolved row)?" — the SPEC-section detection seam consumed by `plan-phase` Step 7.95 (the spec-less probe fallback) to decide, per section, whether to run the fallback. Replaces the ad-hoc `awk` that previously lived in the workflow body, which hard-coded the section header strings at the call site and hand-rolled markdown-table row counting — a brittleness that produced two bugs: an exact `^## Prohibitions$` anchor that missed the canonical `## Prohibitions (must-NOT)` heading, and a single-table row-counting assumption. **Suffix-tolerant header invariant:** `SECTION_HEADERS` regexes match a heading AND any parenthetical/whitespace suffix — `prohibitions` matches both `## Prohibitions` and `## Prohibitions (must-NOT)`; `edges` matches `## Edge Coverage` (and any future suffix); if spec-phase renames a heading, update HERE and the `templates/spec.md` heading together (the contract is pinned by `tests/spec-section.test.cjs`). **Supply rule:** `supplied = present AND dataRows > 0` — a present-but-empty section is NOT supplied (it triggers the fallback). **Multi-table robustness:** a blank or prose line resets the per-table state, so a section with multiple tables (or prose between them) counts every table's data rows without miscounting a second table's header row; the `|…|` line before a `|---|` separator is the table header row and is never counted. **Fail-safe:** a missing/unreadable SPEC file resolves to `present:false` / `supplied:false` (so the fallback fires) rather than throwing. Exports (locked surface): the `SpecSectionKey` type (`edges | prohibitions`), `SECTION_HEADERS` (the canonical header matchers), the `SectionStatus` shape (`{ key, present, dataRows, supplied }`), `countSectionDataRows` (pure `specText → { present, dataRows }`), and `specSectionStatus` (disk-reading wrapper). CLI: `node spec-section.cjs <specFile> <edges|prohibitions>` prints `SectionStatus` JSON — exit 0 on success (an absent file is a valid "not supplied" answer), exit 2 only on a usage error (missing args / bad key). Pure and dependency-free. Source of truth: `gsd-core/bin/lib/spec-section.cjs` (generated from `src/spec-section.cts`, gitignored per ADR-457). Tests: `tests/spec-section.test.cjs`. See Edge Probe Module, Prohibition Probe Module, and `references/specless-probe-fallback.md`.
+
 ### MVP Mode
 Phase-level planning mode that frames work as a vertical slice (UI → API → DB) of one user-visible capability instead of horizontal layers. Resolved at workflow init via the precedence chain: `--mvp` CLI flag → ROADMAP.md `**Mode:** mvp` field → `workflow.mvp_mode` config → false. All-or-nothing per phase (PRD #2826 Q1). Surfaced as `MVP_MODE=true|false` to the planner, executor, verifier, and discovery surfaces (progress, stats, graphify). Canonical parser: `roadmap.cjs` `**Mode:**` field; canonical resolution chain documented in `workflows/plan-phase.md`. Concept index: `references/mvp-concepts.md`.