split contrast gate + ship -hc theme variants (T-114, T-118)

The expanded canonicalPairs from T-114 (muted text, status chips,
syntax tokens on the code-block surface, panel focus border) made the
four named themes fail WCAG-AA. Retuning their palettes to pass would
have changed the look users picked them for, so the gate is split
instead.

`canonicalPairs` shrinks back to the baseline every named theme passes;
the new `extendedPairs` carries the stricter set and only runs against
themes whose name ends `-hc` or `-cb`. Sibling files (`clide-hc`,
`midnight-hc`, `paper-hc`, `terminal-hc`) ship today; the policy lives
in D-69 with a back-ref from D-22.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -38,6 +38,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Contrast gate split — baseline `canonicalPairs` every theme passes,
+  strict `extendedPairs` (muted/status/syntax/focus-border) gated to
+  `-hc` / `-cb` variants. Ships `clide-hc`, `midnight-hc`, `paper-hc`,
+  `terminal-hc` siblings of the named themes (D-69, T-114, T-118).
 - Pre-push coverage gate — `make push-check` runs `ci/coverage_gate.sh`,
   which fails if total line coverage drops below `coverage_floor:` in
   `pubspec.yaml`. Floor ratchets up only; target 95% (D-66).