refactor(#1390): migrate check-command-router + gap-checker onto markdown-sectionizer seam (epic #1372 T3) (#1392)

- check-command-router: stripCommentsAndFences delegates fenced-code
  stripping to seam's stripFencedCode; HTML-comment stripping stays
  caller-side. extractPlanDesignatedSections replaces hand-rolled
  split(/\r?\n/) + /^#{1,6}\s+/ heading walk with collectSections
  driven by DESIGNATED_HEADINGS_RE.

- gap-checker: parseRequirements checkbox-bullet detection migrates
  to iterateBullets (checkbox markers); **ID** extracted caller-side.
  Table-row path and separator-row skip stay caller-side.

- Adds refactor-1390-t3-characterization.test.cjs (43 behavioral
  tests) that were green before and remain green after.

- T1 fail-loud / could-not-parse gate semantics unchanged (verified
  by decisions.test.cjs 59/59 and direct gate invocation).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/check-command-router.cts

@@ -20,6 +20,7 @@ import phaseLocatorMod = require('./phase-locator.cjs');
 const { findPhaseInternal } = phaseLocatorMod;
 import { extractDecisions } from './decisions.cjs';
 import type { Decision } from './decisions.cjs';
+import { stripFencedCode, collectSections } from './markdown-sectionizer.cjs';
 import { checkUiPresence } from './ui-safety-gate.cjs';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import verifyModule = require('./verify.cjs');
@@ -134,10 +135,11 @@ const DESIGNATED_HEADINGS_RE = /^#{1,6}\s+(?:must[_ ]haves?|truths?|tasks?|objec
 const XML_DECISION_TAGS_RE = /<(?:objective|tasks?|action)(?:\s[^>]*)?>([\s\S]*?)<\/(?:objective|tasks?|action)>/gi;
 
 function stripCommentsAndFences(text: string): string {
-  return text
-    .replace(/<!--[\s\S]*?-->/g, ' ')
-    .replace(/```[\s\S]*?```/g, ' ')
-    .replace(/~~~[\s\S]*?~~~/g, ' ');
+  // HTML-comment stripping stays caller-side (the seam does not strip HTML comments).
+  const htmlStripped = text.replace(/<!--[\s\S]*?-->/g, ' ');
+  // Fenced-code stripping: delegate to the canonical CommonMark-correct seam.
+  // replaces the prior independent regex copy (```` ``` ``` ````  + `~~~ ~~~`).
+  return stripFencedCode(htmlStripped).text;
 }
 
 function extractYamlBlock(frontmatter: string, key: string): string {
@@ -174,16 +176,18 @@ function extractPlanDesignatedSections(planContent: string | null | undefined):
     if (block) parts.push(block);
   }
 
+  // Replace hand-rolled split(/\r?\n/) + heading walk with the seam's collectSections.
+  // stopPredicate fires on EVERY heading (collectSections needs to start a section at
+  // each heading), then we filter to designated ones — same semantics as the prior
+  // inDesignated flag: emit the heading line + body only when DESIGNATED_HEADINGS_RE matches.
+  const sections = collectSections(body, () => true);
   const bodyParts: string[] = [];
-  let inDesignated = false;
-  for (const line of body.split(/\r?\n/)) {
-    const heading = /^#{1,6}\s+/.test(line);
-    if (heading) {
-      inDesignated = DESIGNATED_HEADINGS_RE.test(line);
-      if (inDesignated) bodyParts.push(line);
-      continue;
+  for (const section of sections) {
+    const headingLine = '#'.repeat(section.heading.level) + ' ' + section.heading.text;
+    if (DESIGNATED_HEADINGS_RE.test(headingLine)) {
+      bodyParts.push(headingLine);
+      if (section.body) bodyParts.push(section.body);
     }
-    if (inDesignated) bodyParts.push(line);
   }
   parts.push(bodyParts.join('\n'));
   parts.push(extractXmlTagBodies(cleaned));

src/gap-checker.cts

@@ -28,6 +28,7 @@ const { escapeRegex } = phaseId;
 import planningWorkspace = require('./planning-workspace.cjs');
 const { planningPaths, planningDir, findContextMdIn } = planningWorkspace;
 import { parseDecisions, extractDecisions } from './decisions.cjs';
+import { iterateBullets } from './markdown-sectionizer.cjs';
 
 // ─── Types ────────────────────────────────────────────────────────────────────
 
@@ -81,18 +82,26 @@ function parseRequirements(reqMd: unknown): ReqItem[] {
 
   // Prefix-agnostic ID format: REQ-01, TST-01, BACK-07, INSP-04, etc.
   const ID_PATTERN = '[A-Z][A-Z0-9]*-[A-Za-z0-9_-]+';
+  const idRe = new RegExp(`^(${ID_PATTERN})$`);
 
-  const checkboxRe = new RegExp(`^\\s*-\\s*\\[[x ]\\]\\s*\\*\\*(${ID_PATTERN})\\*\\*\\s*(.*)$`, 'gm');
-  let cm = checkboxRe.exec(reqMd);
-  while (cm !== null) {
-    const id = cm[1];
+  // Checkbox-bullet path: migrate to seam's iterateBullets (checkbox markers).
+  // The **ID** is extracted from the bullet text caller-side — the seam provides
+  // the raw text; we parse the bold-ID prefix from it here.
+  const boldIdRe = new RegExp(`^\\*\\*(${ID_PATTERN})\\*\\*\\s*(.*)$`);
+  for (const bullet of iterateBullets(reqMd)) {
+    if (bullet.marker !== 'checkbox-unchecked' && bullet.marker !== 'checkbox-checked') continue;
+    const m = boldIdRe.exec(bullet.text);
+    if (!m) continue;
+    const id = m[1];
+    if (!idRe.test(id)) continue;
     if (!seen.has(id)) {
       seen.add(id);
-      out.push({ id, text: (cm[2] || '').trim() });
+      out.push({ id, text: (m[2] || '').trim() });
     }
-    cm = checkboxRe.exec(reqMd);
   }
 
+  // Pipe-table-row path and separator-row skip stay caller-side
+  // (table parsing is out of seam scope per ADR-1372 T3 spec).
   const tableFirstCellRe = new RegExp(`^\\s*\\|\\s*(${ID_PATTERN})\\s*\\|`);
   const separatorRowRe = /^\s*\|[\s:|-]+\|\s*$/;
   const lines = reqMd.split(/\r?\n/);