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 Historical
  references preserved (e.g. "Was cyan #6fc2ef before the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/ui_modal.rs

@@ -333,17 +333,17 @@ pub fn spawn_modal_button<M: Component>(
     };
 
     let label_color = match variant {
-        // Primary buttons sit on the cyan accent — `BG_BASE` text on
-        // top reads well and passes AAA contrast against `#6fc2ef`.
+        // Primary buttons sit on the brick-red accent — `BG_BASE` text on
+        // top reads well and passes AAA contrast against `#a54242`.
         ButtonVariant::Primary => BG_BASE,
         ButtonVariant::Secondary | ButtonVariant::Tertiary => TEXT_PRIMARY,
     };
     let caption_color = match variant {
-        // Muted near-black on the cyan Primary so the hotkey chip reads
+        // Muted near-black on the red Primary so the hotkey chip reads
         // as a secondary detail without disappearing. Deliberately a
         // pure-black-at-alpha rather than `BG_BASE.with_alpha(...)`:
-        // `BG_BASE` is `#151515` (not 0,0,0), so the alpha-on-cyan
-        // composite would tint slightly cooler than intended here.
+        // `BG_BASE` is `#151515` (not 0,0,0), so the alpha-on-accent
+        // composite would tint slightly off from intended here.
         ButtonVariant::Primary => Color::srgba(0.0, 0.0, 0.0, 0.55),
         ButtonVariant::Secondary | ButtonVariant::Tertiary => TEXT_SECONDARY,
     };