feat(engine): add ui_modal primitive (scaffold + button variants)

Phase 3 step 3 of the UX overhaul. Adds a reusable modal helper that
the next 6 commits use to convert each overlay screen. The audit found
11 overlays using 3 different visual styles with scrim alpha drift
between 0.60 and 0.92; this primitive collapses all of that into one
consistent shape.

API surface:
- spawn_modal(commands, plugin_marker, z, build_card)  — full-screen
  scrim (uniform SCRIM token) + centred card (BG_ELEVATED, RADIUS_LG,
  BORDER_STRONG outline, max-width 720, min-width 360, padding
  SPACE_5).  Returns the scrim entity for one-call despawn.
- spawn_modal_header(parent, title, font_res)          — TYPE_HEADLINE
  + TEXT_PRIMARY, the canonical overlay heading.
- spawn_modal_body_text(parent, text, color, font_res) — TYPE_BODY_LG
  paragraph; pass TEXT_PRIMARY or TEXT_SECONDARY.
- spawn_modal_actions(parent, build_buttons)           — flex-row
  justify-end with margin-top.
- spawn_modal_button(parent, marker, label, hotkey,
                     variant, font_res)                — real Button
  entity with optional TYPE_CAPTION hotkey-hint chip.

ButtonVariant enum drives colour:
  Primary    idle ACCENT_PRIMARY      hover ACCENT_PRIMARY_HOVER
             pressed ACCENT_SECONDARY (yellow → pink press flash)
  Secondary  idle BG_ELEVATED_HI      hover BG_ELEVATED_TOP
             pressed BG_ELEVATED
  Tertiary   idle BG_ELEVATED         hover BG_ELEVATED_HI
             pressed BG_ELEVATED_PRESSED

A new BG_ELEVATED_TOP token plus ACCENT_PRIMARY_HOVER cover the new
hover/press combinations cleanly.

UiModalPlugin registers paint_modal_buttons so every ModalButton gets
hover and press feedback automatically — overlay plugins don't add
their own paint systems. Plugin registered in solitaire_app.

A self-test asserts each variant's idle / hover / pressed colours are
all distinct; another verifies the plugin builds under MinimalPlugins.

This commit is purely additive — no overlay calls the new helpers
yet. The next commits convert each overlay to use them.

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

solitaire_engine/src/ui_theme.rs

@@ -36,6 +36,10 @@ pub const BG_ELEVATED: Color = Color::srgb(0.176, 0.106, 0.412);
 /// currently-active row of a popover. `#3A2580`.
 pub const BG_ELEVATED_HI: Color = Color::srgb(0.227, 0.145, 0.502);
 
+/// Top elevation step — Secondary button hover, popover currently-
+/// hovered row. One rung above `BG_ELEVATED_HI`. `#482F97`.
+pub const BG_ELEVATED_TOP: Color = Color::srgb(0.282, 0.184, 0.592);
+
 /// Pressed-button surface — `BG_ELEVATED` darkened ~15%. `#26155B`.
 pub const BG_ELEVATED_PRESSED: Color = Color::srgb(0.149, 0.082, 0.357);
 
@@ -61,6 +65,10 @@ pub const TEXT_DISABLED: Color = Color::srgb(0.420, 0.373, 0.522);
 /// AAA contrast. `#FFD23F`.
 pub const ACCENT_PRIMARY: Color = Color::srgb(1.000, 0.824, 0.247);
 
+/// Brightened `ACCENT_PRIMARY` for hover states on primary buttons.
+/// Picks up saturation while keeping the same hue. `#FFE36B`.
+pub const ACCENT_PRIMARY_HOVER: Color = Color::srgb(1.000, 0.890, 0.420);
+
 /// Warm magenta secondary accent — celebratory states (achievement
 /// unlocked, streak milestones). Used sparingly so it stays special.
 /// `#FF6B9D`.