fix(scripts): stop the optional-error-sink gate's own header lying about its narrowing count (#12323)

:60 and :789 said "both"/"the two narrowings" while the file documents
and pins three (Narrowing 1: as-cast, Narrowing 2: purity, Narrowing 3:
unreadable member type, added by #11069) and its runtime string already
prints "all three narrowings pinned as counts". Repointed both sentences
to an integer-free spelling ("every narrowing below" / "the narrowings")
so they cannot go stale again when a fourth narrowing arrives, matching
the fix PR #12290 made for the sibling workflow-comment instance.

:985 is left exactly as written — it is a correct past-tense statement
about the state of the world before narrowing 3 existed (#11069's own
history), not a present-tense claim.

No behaviour, count, pin, or runtime string changed.

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

Author: yinlianghui

scripts/check-optional-error-sink-contract.mjs

@@ -57,8 +57,8 @@
  * misses a type someone names differently, and this repo has `LoggerLike`,
  * `MinimalLogger`, `OptionalLogger`, `FakeLogSink` and eight anonymous inline
  * literals — a name rule would have missed most of the population. Structural
- * costs false positives instead, so both narrowings below are measured and
- * their cost is printed on every run rather than argued in prose.
+ * costs false positives instead, so every narrowing below is measured and
+ * its cost is printed on every run rather than argued in prose.
  *
  * A SINK is: an interface, a type alias to a type literal, or an inline type
  * literal, all of whose members are function-typed and named from
@@ -786,7 +786,7 @@ function run({ list = false } = {}) {
 // ── Self-test ───────────────────────────────────────────────────────────────
 
 /**
- * Every limb observed FAILING and observed SILENT, plus the two narrowings
+ * Every limb observed FAILING and observed SILENT, plus the narrowings
  * pinned as positive counts.
  *
  * The reject side is pinned deliberately: a matcher that stopped matching