feat(engine): revert to traditional 2-colour deck with saturated red + near-white

Per player feedback after the brief 4-colour-deck experiment:
"can we make the card suit colors the same as a regular
solitaire game would." Reverts the 4-colour split (`62b61cc`)
and bumps both 2-colour hues to read more like a real
Microsoft-Solitaire-on-dark-mode deck.

### Constants

- `RED_SUIT_COLOUR`: `#fb9fb1` (Terminal pink, then briefly
  hearts-only) → `#e35353` (saturated red). More chromatic, less
  pastel; reads as "the red suit" rather than "a Terminal-
  themed pink." Visually distinct from `ACCENT_PRIMARY`
  `#a54242` (the brick-red CTA accent) so chrome and suit don't
  collapse to the same hue.
- `BLACK_SUIT_COLOUR`: `#d0d0d0` (matched `TEXT_PRIMARY`) →
  `#e8e8e8` (near-white). Bumped slightly brighter so it reads
  as a chromatic-neutral counterpart to the new saturated red,
  not as "the same gray as body text." `TEXT_PRIMARY_HC`
  (`#f5f5f5`) is still brighter for the high-contrast boost
  path.
- `RED_SUIT_COLOUR_HC`: `#ff8aa0` (pinkish boost matching the
  v0.21.0 pink default) → `#ff6868` (brighter saturated red).
  Now reads as "more chromatic" than the new default red, not
  "less saturated."
- `DIAMOND_SUIT_COLOUR` and `CLUB_SUIT_COLOUR` deleted — the
  4-colour split is gone, hearts/diamonds re-pair under
  `RED_SUIT_COLOUR` and clubs/spades under
  `BLACK_SUIT_COLOUR`.

### `card_face_svg.rs`

- Module-level constants collapse from four (`SUIT_HEART` /
  `SUIT_DIAMOND` / `SUIT_CLUB` / `SUIT_SPADE`) back to two
  (`SUIT_RED` / `SUIT_DARK`) at the new saturated-red /
  near-white values.
- `suit_paint()` reverts to the 2-colour pairing: hearts
  filled-red, diamonds outlined-red, spades filled-near-white,
  clubs outlined-near-white. Filled-vs-outlined glyph
  differentiation stays the always-on CBM fallback.

### `card_plugin.rs`

- `text_colour()` reverts to a `card.suit.is_red()`
  bifurcation. Comment block updated to reflect the new
  truth table: red suits → saturated red (or CBM lime / HC
  brighter red); dark suits → near-white (or HC brighter
  near-white).

### Tests

Test block restructured back to the pre-4-colour shape: two
red/black pairing tests instead of one 4-colour distinctness
test. CBM/HC compose tests retuned to the 2-colour world (red
suits compose, dark suits compose; no separate diamonds-immune
or clubs-immune cases). 1191 passing / 0 failing — net 0 from
the prior commit (3 tests removed: the 4-colour distinctness
test + the diamonds/clubs-immune test; 2 tests added back: the
red-pairing + dark-pairing tests; existing tests amended to
new colour assumptions).

### `card_face_svg_pin`

All 52 face hashes drift (every suit's colour shifted); 5 back
hashes unchanged. Surgical rebaseline.

### `design-system.md`

§Suit Colors retitled "Two-color traditional pairing", table
updated with the new hex values, CBM section text simplified
back to red→lime swap on both red suits.

Workspace clippy clean.

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

solitaire_engine/assets/themes/default/hearts_6.svg

@@ -1,18 +1,18 @@
 <svg xmlns="http://www.w3.org/2000/svg" width="256" height="384" viewBox="0 0 256 384">
   <rect x="1" y="1" width="254" height="382" rx="16" ry="16"
-        fill="#1a1a1a" stroke="#fb9fb1" stroke-width="2"/>
+        fill="#1a1a1a" stroke="#e35353" stroke-width="2"/>
 
   <!-- Top-left rank in JetBrains-Mono-styled FiraMono (rank digits
        and letters render correctly in FiraMono; only the suit glyphs
        needed to escape to paths). -->
   <text x="14" y="44" font-family="Fira Mono" font-size="36" font-weight="700"
-        fill="#fb9fb1">6</text>
+        fill="#e35353">6</text>
 
   <!-- Top-left small suit glyph at (14, 50), 20 × 20.
        `suit_path_d` is authored in a 32-unit box, so scale 0.625
        lands the visible glyph at 20 px. -->
   <g transform="translate(14 50) scale(0.625)">
-    <path d="M16,28 C 8,22 2,17 2,11 C 2,7 5,4 9,4 C 12,4 14,6 16,9 C 18,6 20,4 23,4 C 27,4 30,7 30,11 C 30,17 24,22 16,28 Z" fill="#fb9fb1"/>
+    <path d="M16,28 C 8,22 2,17 2,11 C 2,7 5,4 9,4 C 12,4 14,6 16,9 C 18,6 20,4 23,4 C 27,4 30,7 30,11 C 30,17 24,22 16,28 Z" fill="#e35353"/>
   </g>
 
   <!-- Bottom-right large suit glyph at (178, 286), 64 × 64.
@@ -20,6 +20,6 @@
        (178, 286). Same upright orientation as the top-left small
        glyph — no 180° rotation applied. -->
   <g transform="translate(178 286) scale(2)">
-    <path d="M16,28 C 8,22 2,17 2,11 C 2,7 5,4 9,4 C 12,4 14,6 16,9 C 18,6 20,4 23,4 C 27,4 30,7 30,11 C 30,17 24,22 16,28 Z" fill="#fb9fb1"/>
+    <path d="M16,28 C 8,22 2,17 2,11 C 2,7 5,4 9,4 C 12,4 14,6 16,9 C 18,6 20,4 23,4 C 27,4 30,7 30,11 C 30,17 24,22 16,28 Z" fill="#e35353"/>
   </g>
 </svg>