docs(check-release-section-coverage): mark the v17 fixture historical, not "actually shipped"

The `REAL_V17_HEADINGS` fixture carried a present-tense docblock — "Kept
verbatim so the matcher is judged against text that actually shipped" — while
the text it snapshots no longer ships. #13754 (merged 2026-08-31) cascaded the
release-page headings: `content/docs/releases/v17.mdx` now reads
`## 17.0.0 in detail`, and no page under `content/docs/releases/` carries a
body-level h1 any more (0 such lines across the ten pages, counter-checked
against 54 `## ` lines in the same files).

The gate is green either way and stays green — `headingsNamingMinor` is
`^#{1,6}`, level-agnostic by construction, and nothing compares the fixture to
the live page. The defect is the claim, not the behaviour: the file already
keeps a deliberately historical snapshot beside it (`PRE_10232_V17_HEADINGS`),
so a reader diffing either against the live page could not tell which was
which.

Relabelled to that sibling's existing convention rather than a third one:
renamed `REAL_V17_HEADINGS` -> `PRE_13754_V17_HEADINGS` (the identifier is the
label a reader meets at the three use sites; the docblock is only visible at
the definition), and the docblock now names what changed and when, as
`PRE_10232_V17_HEADINGS`'s does.

The docblock also carries the reason it must not be re-snapshotted, measured
rather than assumed: a re-snapshot would remove the last h1 that reaches the
matcher through a whole-page fixture, and would remove it in silence — 17.0 is
covered twice over here, so every case in this file stays green with the `# `
demoted to `##`. The matcher's `^#{1,6}` breadth itself is pinned separately by
the `'# 16.1.0'` / `'###### 16.1.0'` case, so the fixture is NOT the only
h1-bearing input and the docblock does not claim to be.

The 17.0 self-test label said the fixture covers 17.0 "via `# 17.0.0 in
detail`", which reads as though the h1 were load-bearing there. Measured: 17.0
matches both `## Highlights — 17.0.0` and `# 17.0.0 in detail`. The label now
says so, and says the case cannot catch a re-snapshot.

No changeset: script-only, publishes nothing from any package — the repo's
precedent (15/15 recent commits touching only `scripts/check-*.mjs`) carries
none. `skip-changeset` applied on the PR.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

Author: claude

scripts/check-release-section-coverage.mjs

@@ -370,10 +370,25 @@ export function indexCurrencyFindings(major, newestVersion, line) {
 // ── Self-test ────────────────────────────────────────────────────────────────
 
 /**
- * The real v17 page heading set, trimmed to the lines that matter. Kept verbatim
- * so the matcher is judged against text that actually shipped.
+ * The real v17 page heading set as it stood between #10232 and #13754, trimmed to
+ * the lines that matter. HISTORICAL — a snapshot, like `PRE_10232_V17_HEADINGS`
+ * below, and no longer the page as it reads today: #13754 (merged 2026-08-31)
+ * cascaded the release-page headings, so this page's `# 17.0.0 in detail` is now
+ * `## 17.0.0 in detail` and NO page under `content/docs/releases/` carries a
+ * body-level h1 any more (measured on the merge commit: 0 such lines across the
+ * ten pages, against 54 `## ` lines in the same files — so the zero is a reading,
+ * not a broken query).
+ *
+ * Do NOT re-snapshot it from the live page. `headingsNamingMinor` is `^#{1,6}` by
+ * construction and its breadth is pinned directly by the `'# 16.1.0'` /
+ * `'###### 16.1.0'` case in the self-test below, which a re-snapshot would not
+ * touch. What a re-snapshot WOULD remove is the last h1 reaching the matcher
+ * through a whole-PAGE fixture — and it would remove it in silence: 17.0 is
+ * covered here twice over (`## Highlights — 17.0.0` matches it too), so every case
+ * in this file stays green with the `# ` demoted to `##`. Measured, not assumed.
+ * The ban is written here because nothing mechanical enforces it.
  */
-const REAL_V17_HEADINGS = [
+const PRE_13754_V17_HEADINGS = [
   '## Highlights — 17.0.0',
   '## Highlights — 17.1.0',
   '# 17.0.0 in detail',
@@ -544,13 +559,14 @@ export function selfTest() {
       .some((f) => f.includes('NO heading on this page names the 17.1 series')),
   );
   expect(
-    'coverage/GREEN — the real v17 headings AFTER #10232 cover 17.1 via "## Highlights — 17.1.0". A '
-    + 'check that reds on everything is not a check',
-    coverageFindings(17, 1, 'v17.mdx', REAL_V17_HEADINGS).length === 0,
+    'coverage/GREEN — the real v17 headings between #10232 and #13754 cover 17.1 via "## Highlights '
+    + '— 17.1.0". A check that reds on everything is not a check',
+    coverageFindings(17, 1, 'v17.mdx', PRE_13754_V17_HEADINGS).length === 0,
   );
   expect(
-    'coverage/GREEN — and they cover 17.0 via "# 17.0.0 in detail"',
-    coverageFindings(17, 0, 'v17.mdx', REAL_V17_HEADINGS).length === 0,
+    'coverage/GREEN — and they cover 17.0 TWICE OVER, via "## Highlights — 17.0.0" and "# 17.0.0 in '
+    + 'detail". Neither is load-bearing alone, so this case cannot catch a re-snapshot of the h1',
+    coverageFindings(17, 0, 'v17.mdx', PRE_13754_V17_HEADINGS).length === 0,
   );
   expect(
     'coverage — TRAP 1: `## What\'s new in 11.10.0` does NOT cover minor 11.1. Major 11 published '