feat(engine): swap ACCENT_PRIMARY from cyan #6fc2ef to brick red #a54242

Project-wide palette shift at user request. Replaces the cyan
primary accent everywhere it surfaces — splash boot screen,
home menu glyphs, action chevrons, replay overlay banner +
scrub fill + chip border, achievement checkmarks, leaderboard
#1 indicator, radial menu fill, focus ring, card-back canonical
badge, etc. — with `#a54242` from the same base16-eighties
family as the existing pink suit colour.

Knock-on changes that all land in this commit per the
lockstep rule:

- ui_theme.rs: ACCENT_PRIMARY (#a54242), ACCENT_PRIMARY_HOVER
  (#c25e5e brightened companion), FOCUS_RING (same hue, 0.85
  alpha). Module-level palette comment + STOCK_BADGE_FG +
  CARD_SHADOW_ALPHA_DRAG doc strings updated to match.
- card_plugin.rs: card_back_colour(0) now returns the brick-red
  ACCENT_PRIMARY (was cyan). RED_SUIT_COLOUR_CBM swapped from
  cyan to lime #acc267 — the CBM alternative needs to stay
  hue-distinct from the new red-family primary, lime is the
  next-best non-red base16-eighties accent. text_colour doc
  + CBM tests renamed cyan→lime in lockstep
  (text_colour_color_blind_mode_swaps_red_suits_to_lime).
- card_face_svg.rs: BACK_ACCENTS[0] now "#a54242" (canonical
  Terminal back).
- splash_plugin.rs / ui_modal.rs / replay_overlay.rs /
  selection_plugin.rs: descriptive "cyan" comments swapped to
  "accent" / "primary-accent" wording so the doc strings stay
  decoupled from any specific hue. Future palette tweaks won't
  require comment churn.
- design-system.md: YAML token frontmatter updated (primary,
  surface-tint, suit-red-cb, primary-container,
  on-primary-container, inverse-primary). Palette table gains
  a project-specific `base08` slot for the new red. CTA /
  Selection / Card-back badge / Primary button / Bottom-bar
  active-icon / glow / CBM swap text all retuned. Historical
  references preserved (e.g. "Was cyan #6fc2ef before the
  2026-05-08 swap") so the audit trail stays in the spec.
- card_face_svg_pin.rs: rebaselined. Exactly one hash drift
  (back_0 — the canonical Terminal back's badge changed
  colour). Other 56 hashes identical (face SVGs don't
  reference the accent; back_1..4 use unchanged accents). The
  one-hash-drift signal confirms the change scope was
  surgical.

Workspace clippy + cargo test --workspace clean, 1184 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/card_plugin.rs

@@ -98,12 +98,18 @@ pub struct CardImageSet {
 }
 
 /// Suit-colour swap for red-suit cards in colour-blind mode — Terminal
-/// `#6fc2ef` (cyan). Replaces `RED_SUIT_COLOUR` (pink) when CBM is on,
+/// `#acc267` (lime). Replaces `RED_SUIT_COLOUR` (pink) when CBM is on,
 /// providing a hue-distinct alternative that survives the most common
 /// red/green deficiencies. Pre-Terminal this was a *face tint*; the new
 /// design moves CBM differentiation into the suit glyph colour itself
 /// and keeps the face uniformly `CARD_FACE_COLOUR` regardless of CBM.
-const RED_SUIT_COLOUR_CBM: Color = Color::srgb(0.435, 0.761, 0.937);
+///
+/// The CBM swap is lime (not the `ACCENT_PRIMARY` brick-red) because
+/// the primary accent is itself in the red family — using it for
+/// "the not-red CBM alternative" would defeat the purpose. Lime is
+/// the next-best non-red base16-eighties accent; deuteranopia and
+/// protanopia readers see it as visibly distinct from pink.
+const RED_SUIT_COLOUR_CBM: Color = Color::srgb(0.675, 0.761, 0.404);
 
 /// Returns the fallback card-back colour for the given unlocked card-back
 /// index. Production renders backs from PNG artwork; this fallback only
@@ -112,7 +118,7 @@ const RED_SUIT_COLOUR_CBM: Color = Color::srgb(0.435, 0.761, 0.937);
 /// in the same hue family as the on-disk PNG art for that index.
 fn card_back_colour(selected_card_back: usize) -> Color {
     match selected_card_back {
-        0 => Color::srgb(0.435, 0.761, 0.937), // #6fc2ef cyan (Terminal canonical)
+        0 => Color::srgb(0.647, 0.259, 0.259), // #a54242 brick red (Terminal canonical, ACCENT_PRIMARY)
         1 => Color::srgb(0.675, 0.761, 0.404), // #acc267 lime
         2 => Color::srgb(0.882, 0.639, 0.933), // #e1a3ee lavender
         3 => Color::srgb(0.984, 0.624, 0.694), // #fb9fb1 pink
@@ -792,11 +798,15 @@ fn label_for(card: &Card) -> String {
 /// Suit colour for the rank/suit overlay rendered atop the constant
 /// fallback sprite (only fires under `MinimalPlugins` — production
 /// renders the suit glyph baked into the PNG). When `color_blind` is
-/// enabled, red-suit cards swap to `RED_SUIT_COLOUR_CBM` (cyan) — the
-/// "Settings toggle swaps red→cyan" half of the design system's
+/// enabled, red-suit cards swap to `RED_SUIT_COLOUR_CBM` (lime) — the
+/// "Settings toggle swaps red→lime" half of the design system's
 /// colour-blind support. The other half (always-on filled-vs-outlined
 /// glyph differentiation for ♥♠ vs ♦♣) is baked into the PNG art and
 /// has no constant-fallback equivalent.
+///
+/// The CBM swap is lime (not the new brick-red `ACCENT_PRIMARY`)
+/// because the primary accent is itself red-family — the CBM
+/// alternative needs to be hue-distinct from the original red suit.
 fn text_colour(card: &Card, color_blind: bool) -> Color {
     if card.suit.is_red() {
         if color_blind {
@@ -2066,20 +2076,20 @@ mod tests {
     // Pre-Terminal these were `face_colour` tests asserting that CBM
     // tinted the *face background* of red-suit cards. The Terminal
     // design system moves CBM differentiation into the suit *glyph*
-    // colour (red→cyan), so these tests now exercise `text_colour`.
+    // colour (red→lime), so these tests now exercise `text_colour`.
     // -----------------------------------------------------------------------
 
     #[test]
-    fn text_colour_color_blind_mode_swaps_red_suits_to_cyan() {
+    fn text_colour_color_blind_mode_swaps_red_suits_to_lime() {
         let red_card = Card { id: 0, suit: Suit::Diamonds, rank: Rank::Queen, face_up: true };
         let cbm_colour = text_colour(&red_card, true);
         assert_eq!(
             cbm_colour, RED_SUIT_COLOUR_CBM,
-            "color-blind mode must replace the red suit colour with the CBM cyan",
+            "color-blind mode must replace the red suit colour with the CBM lime",
         );
         assert_ne!(
             cbm_colour, RED_SUIT_COLOUR,
-            "CBM red must be visibly distinct from the default red suit colour",
+            "CBM lime must be visibly distinct from the default red suit colour",
         );
     }