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>

lib/kernel/src/theme/themes/paper-hc.yaml

@@ -0,0 +1,38 @@
+# paper-hc — high-contrast sibling of `paper`
+#
+# Drafting sheet, red-pencil accent — darker ink so muted text, status
+# chips, syntax tokens, and the focus border clear the strict contrast
+# gate (D-22 + D-69, `extendedPairs` in lib/kernel/src/theme/contrast.dart).
+
+name: paper-hc
+display_name: Paper (high contrast)
+dark: false
+
+palette:
+  background: "#F4F1EA"
+  panel: "#ECE7DB"
+  surface: "#FBF8F1"
+  muted: "#4A4A48"
+  foreground: "#0E0E0E"
+  secondary: "#1F1F1F"
+  primary: "#8E2D10"
+  accent: "#6E2410"
+  success: "#1A4A2B"
+  warning: "#6A4C00"
+  error: "#7A1F12"
+  info: "#154168"
+  surfaceHi: "#ECE7DB"
+  border: "#0E0E0E"
+  borderHi: "#3A3A38"
+  textDim: "#4A4A48"
+  textMute: "#7A7A72"
+  accentSoft: "#218E2D10"
+
+syntax:
+  keyword: "#52215C"
+  type: "#154168"
+  string: "#1A4A2B"
+  number: "#6A4C00"
+  comment: "#4A4A48"
+  method: "#103E6F"
+  punct: "#1F1F1F"