docs(spec): use the yes (narrowing) arm, the spelling for a diff that does both

The changeset declared `yes (widening)`. `yes` is right — a new exported symbol
is the mechanical floor — but the arm was not: this diff narrows the accept set.

The vocabulary already has the spelling for this. `readClause2Line` accepts
`yes (narrowing)`, and check-adr-0087-registration's own self-test names the
case verbatim: "the `narrowing` arm beside a `yes` value — a diff that widens
AND narrows". The earlier wording needed a paragraph explaining why `widening`
did not mean what it says; the correct arm needs none, and it restores the
`clause-②-narrowing` signal the gate reads.

`no (widening)` stays malformed, so the arms are not free: `no` takes only
`narrowing`, while `yes` takes either.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JbZnqu8bt6YqfJsr9vaFb3

Author: claude

.changeset/17779-dashboard-metric-family-single-measure.md

@@ -4,7 +4,7 @@
 
 feat(spec)!: a metric-family dashboard widget declares exactly ONE measure — `values` is bounded above on `metric` / `kpi` / `gauge` / `solid-gauge` / `bullet` (#17779; objectui#8894 ruling D, decision batch #119 item 4)
 
-Clause-②: yes (widening) — routing, not direction. The accept set NARROWS (that is this change). What makes the line `yes` is the other axis: the published surface GAINS one exported symbol, `checkDashboardWidgetMetricMeasureArity`, and a new exported symbol is the mechanical floor for in-seat contract review. Breaking-ness is carried by the **BREAKING** banner and the ADR-0087 disposition above, not by this line.
+Clause-②: yes (narrowing) — this diff BOTH narrows and widens, which is the shape this arm exists for. The accept set NARROWS (that is the change). What makes the value `yes` is the other axis: the published surface GAINS one exported symbol, `checkDashboardWidgetMetricMeasureArity`, and a new exported symbol is the mechanical floor for in-seat contract review.
 
 <!-- adr-0087: registered dashboard-widget-metric-family-multi-measure-refused -->