feat(engine): add ui_theme.rs design-token module

Phase 3 step 1 of the UX overhaul. Centralises every UI design token —
colours, typography, spacing, border-radius, z-index, and motion
durations — so subsequent overhaul commits read from one source of
truth instead of scattering hex codes and magic numbers across plugin
files.

The audit (2026-04-30) found:
- 40+ hardcoded Color::srgb literals across UI surfaces.
- 12 distinct font sizes (14/15/16/17/18/22/26/28/30/32/40/48 px)
  with no scale.
- 8+ z-index magic numbers across overlay plugins (200, 210, 220,
  230, 250, 300, 400) with no documented hierarchy.
- Motion durations only partially honouring AnimSpeed — slide and
  cascade did, but toast / shake / settle / deal were hardcoded.

ui_theme.rs collapses these into:
- Midnight Purple base (BG_BASE / BG_ELEVATED / BG_ELEVATED_HI /
  BG_ELEVATED_PRESSED) + Balatro-yellow ACCENT_PRIMARY + warm
  magenta ACCENT_SECONDARY + state colours (success/warning/danger/
  info) + text tiers (primary/secondary/disabled) + a uniform SCRIM.
- 5-rung typography scale (display 40 / headline 26 / body-lg 18 /
  body 14 / caption 11).
- 4-multiple spacing scale (4/8/12/16/24/32/48), with VAL_SPACE_*
  Val::Px convenience constants.
- 3 border-radius rungs (sm 4 / md 8 / lg 16).
- Documented monotonically-increasing z-index hierarchy enforced
  by a unit test.
- All MOTION_* duration constants funnelled through scaled_duration()
  so AnimSpeed (Normal/Fast/Instant) applies to every animation,
  not just slide and cascade.

This commit is purely additive — no call sites change yet.
Subsequent commits in the overhaul migrate plugins to the tokens
one region at a time (HUD restructure, modal primitive, then per-
overlay conversions).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -30,6 +30,7 @@ pub mod stats_plugin;
 pub mod sync_plugin;
 pub mod table_plugin;
 pub mod time_attack_plugin;
+pub mod ui_theme;
 pub mod weekly_goals_plugin;
 pub mod win_summary_plugin;