feat(engine): switch card fronts to 4-colour deck

Hearts pink (`#fb9fb1`), Diamonds gold (`#ddb26f`), Clubs lime
(`#acc267`), Spades gray (`#d0d0d0`) — each suit picks up its
own base16-eighties accent so a player scanning the table can
distinguish the suit by hue alone (faster recognition than the
2-colour traditional red/black scheme; common in poker decks).
All four colours already exist in the palette as semantic
state-token accents, so this is a pure remapping at the suit-
glyph site, not a palette extension.

The outlined-glyph differentiation (♦ ♣ outlined, ♥ ♠ filled)
is preserved on top of the colour split — it stays the always-
on colour-blind fallback per `design-system.md` §Accessibility,
and matters more than ever now that CBM hearts (lime) and
default clubs (lime) share a hue.

### Changes

- `card_face_svg.rs`: split `SUIT_RED` / `SUIT_DARK` into four
  per-suit constants (`SUIT_HEART` / `SUIT_DIAMOND` / `SUIT_CLUB`
  / `SUIT_SPADE`). `suit_paint()` returns each suit's own
  colour. Card border picks up the suit colour automatically
  via the existing `(colour, paint)` destructure.
- `card_plugin.rs`: new `DIAMOND_SUIT_COLOUR` + `CLUB_SUIT_COLOUR`
  constants; `text_colour()` rewritten as a per-suit match (was
  red/black bifurcation). Both rendering paths (PNG production +
  constant fallback under MinimalPlugins) stay in lockstep.
- CBM behaviour clarified: only hearts swap to lime now;
  diamonds + clubs + spades are already hue-distinct from
  the heart pink and stay unchanged. Under CBM the heart
  (lime) and club (lime) share a hue but stay distinguishable
  via the always-on filled-vs-outlined glyph differentiation.
- HC behaviour: only hearts (→ HC red) and spades (→ HC white)
  have defined boosts. Diamonds (gold) and clubs (lime) are
  already mid-luminance accents and stay at their default.
  New test `text_colour_diamonds_and_clubs_are_immune_to_accessibility_flags`
  pins all four flag combinations as no-ops for the gold +
  lime suits.
- `design-system.md` §Suit Colors retitled "Four-color deck"
  with the 4-colour table; CBM section text updated to
  describe the hearts-only swap and the hearts/clubs hue
  collision under CBM.
- `card_face_svg_pin.rs` rebaselined: 26 hashes drift
  (13 clubs + 13 diamonds — the two suits whose colours
  changed). Hearts, spades, and the 5 backs all keep their
  prior hashes. Surgical scope, exactly what the pin test
  was designed to surface.

### Tests

1191 passing / 0 failing — net 0 from the prior baseline:
two old 2-colour tests removed
(`text_colour_is_red_for_hearts_and_diamonds`,
`text_colour_is_black_for_clubs_and_spades`), one consolidated
4-colour test added
(`text_colour_4_colour_deck_assigns_each_suit_its_own_hue`)
plus a pairwise-distinct invariant guard, and one new test
covering the gold/lime suits' immunity to CBM/HC flags. Six
existing CBM/HC tests rewritten to use only the suits each flag
actually affects under the new scheme (hearts for CBM, hearts +
spades for HC).

Workspace clippy clean.

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

docs/ui-mockups/design-system.md

@@ -137,18 +137,22 @@ The palette is base16-eighties — a 16-slot terminal palette where indices 00
 
 ## Suit Colors
 
-**Two-color traditional mapping**, with mandatory color-blind support:
+**Four-color deck**, with mandatory color-blind support. Each suit
+picks up its own base16-eighties accent so a player scanning the
+table can identify the suit by hue alone (faster than the
+traditional 2-color red/black mapping; common in poker decks and
+online card games):
 
 | Suit | Default | Color-blind mode | Glyph differentiation |
 |---|---|---|---|
-| Hearts | `#fb9fb1` (pink) | `#acc267` (lime) | Solid filled glyph |
-| Diamonds | `#fb9fb1` (pink) | `#acc267` (lime) | **Outlined glyph (1.5px stroke)** |
-| Spades | `#d0d0d0` (foreground) | `#d0d0d0` | Solid filled glyph |
-| Clubs | `#d0d0d0` (foreground) | `#d0d0d0` | **Outlined glyph (1.5px stroke)** |
+| Hearts | `#fb9fb1` (pink, base08) | `#acc267` (lime) | Solid filled glyph |
+| Diamonds | `#ddb26f` (gold, base09) | `#ddb26f` (unchanged) | **Outlined glyph (1.5px stroke)** |
+| Spades | `#d0d0d0` (foreground, base05) | `#d0d0d0` (unchanged) | Solid filled glyph |
+| Clubs | `#acc267` (lime, base0A) | `#acc267` (unchanged) | **Outlined glyph (1.5px stroke)** |
 
 The outlined-glyph treatment is the **primary** differentiation mechanism. Color is supplementary. This means a player viewing the game on a monochrome display, or with severe red-green deficiency, can still distinguish all four suits without context. This is a hard requirement, not an optional setting.
 
-The "color-blind mode" toggle in Settings only swaps red→lime; it does not turn the outlined glyphs on or off, because outlined glyphs are always on. (Was red→cyan before the 2026-05-08 primary-accent swap; CBM moved to lime to stay hue-distinct from the new red-family primary.)
+The "color-blind mode" toggle in Settings only swaps the heart suit colour from pink to lime; the other three suits (diamonds gold, clubs lime, spades gray) are already hue-distinct from pink and stay unchanged. The toggle does not turn the outlined glyphs on or off, because outlined glyphs are always on. Note: under CBM with the 4-colour deck, hearts and clubs share the lime hue — the always-on filled-vs-outlined glyph differentiation (♥ filled, ♣ outlined) keeps them readable. (Was red→cyan before the 2026-05-08 primary-accent swap; CBM moved to lime to stay hue-distinct from the new red-family primary.)
 
 ## Typography
 
@@ -272,7 +276,7 @@ Top-right corner of the HUD: a 6px circular dot.
 
 ## Accessibility
 
-1. **Color-blind mode** (Settings → Gameplay): swaps red suits' default `#fb9fb1` for `#acc267` (lime). Outlined-glyph differentiation remains active in *all* modes.
+1. **Color-blind mode** (Settings → Gameplay): swaps the heart suit colour from `#fb9fb1` (pink) to `#acc267` (lime). The other three suits in the 4-colour deck (diamonds gold, clubs lime, spades gray) are already hue-distinct and stay unchanged. Outlined-glyph differentiation remains active in *all* modes.
 2. **High-contrast mode** (Settings → Gameplay): boosts on-surface from `#d0d0d0` to `#f5f5f5`, outline from `#505050` to `#a0a0a0`, suit-red from `#fb9fb1` to `#ff8aa0`.
 3. **Reduce-motion mode** (Settings → Gameplay): disables card-lift transition (instant z-lift), disables CRT scanline effect, disables the warning-chip pulse animation.
 4. **Tabular figures** are mandatory for any number that updates live (timer, score, moves) so they don't reflow.