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/clide-hc.yaml

@@ -0,0 +1,38 @@
+# clide-hc — high-contrast sibling of `clide`
+#
+# Same periwinkle silhouette; muted text, status chips, syntax tokens,
+# and the focus border bumped to clear the strict contrast gate
+# (D-22 + D-69, `extendedPairs` in lib/kernel/src/theme/contrast.dart).
+
+name: clide-hc
+display_name: Clide (high contrast)
+dark: true
+
+palette:
+  background: "#20202C"
+  panel: "#1A1A24"
+  surface: "#242838"
+  muted: "#B1BBE3"
+  foreground: "#E6E8F2"
+  secondary: "#B1BBE3"
+  primary: "#92B5FF"
+  accent: "#92B5FF"
+  success: "#A8E8C8"
+  warning: "#FFD988"
+  error: "#FF9F9F"
+  info: "#A6C8FF"
+  surfaceHi: "#2C3046"
+  border: "#343850"
+  borderHi: "#92B5FF"
+  textDim: "#B1BBE3"
+  textMute: "#9098C0"
+  accentSoft: "#2192B5FF"
+
+syntax:
+  keyword: "#E0B5F2"
+  type: "#A6C8FF"
+  string: "#C2E8B0"
+  number: "#FFD988"
+  comment: "#9098C0"
+  method: "#A6C8FF"
+  punct: "#B1BBE3"

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

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+# midnight-hc — high-contrast sibling of `midnight`
+#
+# Same VS Code-adjacent silhouette; muted text, status chips, syntax
+# tokens, and the focus border bumped to clear the strict contrast
+# gate (D-22 + D-69, `extendedPairs` in lib/kernel/src/theme/contrast.dart).
+
+name: midnight-hc
+display_name: Midnight (high contrast)
+dark: true
+
+palette:
+  background: "#1E1E1E"
+  panel: "#181818"
+  surface: "#252526"
+  muted: "#BDBDBD"
+  foreground: "#F0F0F0"
+  secondary: "#D4D4D4"
+  primary: "#82BFFF"
+  accent: "#82BFFF"
+  success: "#B5E2B0"
+  warning: "#E9D89E"
+  error: "#FFB3A5"
+  info: "#9BC9F0"
+  onAccent: "#0B1220"
+  surfaceHi: "#2D2D2E"
+  border: "#333333"
+  borderHi: "#9C9C9C"
+  textDim: "#BDBDBD"
+  textMute: "#9A9A9A"
+  accentSoft: "#2182BFFF"
+
+syntax:
+  keyword: "#D9A1D3"
+  type: "#66D9C2"
+  string: "#E9B299"
+  number: "#C8E0B7"
+  comment: "#99CB80"
+  method: "#E5E5B5"
+  punct: "#BDBDBD"

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

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+# 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"

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

@@ -0,0 +1,38 @@
+# terminal-hc — high-contrast sibling of `terminal`
+#
+# Near-black + amber tmux feel; muted text, status chips, syntax tokens,
+# and the focus border bumped to clear the strict contrast gate
+# (D-22 + D-69, `extendedPairs` in lib/kernel/src/theme/contrast.dart).
+
+name: terminal-hc
+display_name: Terminal (high contrast)
+dark: true
+
+palette:
+  background: "#0A0A0A"
+  panel: "#000000"
+  surface: "#111111"
+  muted: "#B5B5B5"
+  foreground: "#F0F0F0"
+  secondary: "#D6D6D6"
+  primary: "#FFC868"
+  accent: "#FFC868"
+  success: "#B5E2B0"
+  warning: "#FFD988"
+  error: "#FFA5A5"
+  info: "#B0C8FF"
+  surfaceHi: "#181818"
+  border: "#242424"
+  borderHi: "#888888"
+  textDim: "#B5B5B5"
+  textMute: "#9A9A9A"
+  accentSoft: "#21FFC868"
+
+syntax:
+  keyword: "#FF8585"
+  type: "#FFD27A"
+  string: "#B5E2B0"
+  number: "#DAA7FF"
+  comment: "#9A9A9A"
+  method: "#BCD8FF"
+  punct: "#B5B5B5"