refactor(engine): route gameplay-feedback colours through Terminal tokens

Selection-highlight tints in selection_plugin and the valid-drop
marker tint in cursor_plugin were hand-tuned RGB literals from the
prior Premium-Solitaire palette. Migrate them to the semantic
state tokens introduced in ui_theme:

- keyboard-drag source highlight (picking)  → ACCENT_PRIMARY
- keyboard-drag source highlight (lifted)   → STATE_WARNING
- keyboard-drag destination highlight       → STATE_SUCCESS
- cursor_plugin::MARKER_VALID               → STATE_SUCCESS @ 0.55α

`MARKER_VALID` stays a Color literal (Alpha::with_alpha is not yet
const on stable); a new tracking test pins its RGB to STATE_SUCCESS
so a future palette swap can't drift the two apart silently.

Also fix three stale doc comments in ui_modal that still described
the previous yellow / magenta palette ("Loud yellow CTA",
"Primary swaps to the magenta secondary accent"). Cyan and lavender
now, matching the actual token values.

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

solitaire_engine/src/cursor_plugin.rs

@@ -50,8 +50,17 @@ use crate::ui_theme::{
 /// Kept in sync with the `marker_colour` constant there.
 const MARKER_DEFAULT: Color = Color::srgba(1.0, 1.0, 1.0, 0.08);
 
-/// Green tint applied to pile markers that are valid drop targets during drag.
-const MARKER_VALID: Color = Color::srgba(0.15, 0.85, 0.25, 0.55);
+/// Lime tint applied to pile markers that are valid drop targets during
+/// a drag. Same RGB as the design-system [`STATE_SUCCESS`] token at 55%
+/// alpha, so the in-game "this is a legal target" colour stays
+/// consistent with foundation-completion flourishes and other
+/// valid-move signals. Spelled as a literal because `Alpha::with_alpha`
+/// is not yet a `const` trait method on stable; the tracking test
+/// below pins the RGB to `STATE_SUCCESS` so a palette swap can't drift
+/// the two apart silently. Distinct from [`DROP_TARGET_FILL`] (10%
+/// alpha) because the marker sprite is thin and would otherwise wash
+/// out at a similar opacity.
+const MARKER_VALID: Color = Color::srgba(0.675, 0.761, 0.404, 0.55);
 
 /// Marker component on a parent entity that owns one drop-target overlay
 /// (a translucent fill plus four outline edges as children). The wrapped
@@ -524,6 +533,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn marker_valid_rgb_tracks_state_success_token() {
+        // `MARKER_VALID` is spelled as a literal because
+        // `Alpha::with_alpha` is not a `const` trait method on stable.
+        // This test pins its RGB to `STATE_SUCCESS` so a future
+        // palette swap that updates the token but forgets the marker
+        // fails loudly here.
+        use crate::ui_theme::STATE_SUCCESS;
+        let valid = MARKER_VALID.to_srgba();
+        let success = STATE_SUCCESS.to_srgba();
+        assert!((valid.red - success.red).abs() < 1e-6);
+        assert!((valid.green - success.green).abs() < 1e-6);
+        assert!((valid.blue - success.blue).abs() < 1e-6);
+        assert!((valid.alpha - 0.55).abs() < 1e-6);
+    }
+
     // -----------------------------------------------------------------------
     // pick_cursor_icon priority-order tests
     // -----------------------------------------------------------------------

solitaire_engine/src/selection_plugin.rs

@@ -48,6 +48,7 @@ use crate::input_plugin::{best_destination, best_tableau_destination_for_stack};
 use crate::layout::LayoutResource;
 use crate::pause_plugin::PausedResource;
 use crate::resources::{DragState, GameStateResource};
+use crate::ui_theme::{ACCENT_PRIMARY, STATE_SUCCESS, STATE_WARNING};
 
 // ---------------------------------------------------------------------------
 // Public types
@@ -660,14 +661,18 @@ fn update_selection_highlight(
     };
     let card_size = layout.0.card_size;
 
-    // Choose colours per mode: cyan in source-pick, gold while lifted.
+    // Highlight tints follow the Terminal palette's semantic state
+    // tokens: cyan focus/selection while picking the source, gold
+    // attention/commitment once the cards are lifted, lime valid-move
+    // tint on the destination. Alphas are kept non-zero so the card
+    // face beneath remains readable through the wash.
     let lifted = kbd_drag.is_lifted();
     let source_color = if lifted {
-        Color::srgba(1.0, 0.84, 0.0, 0.6)
+        STATE_WARNING.with_alpha(0.6)
     } else {
-        Color::srgba(0.0, 1.0, 1.0, 0.5)
+        ACCENT_PRIMARY.with_alpha(0.5)
     };
-    let dest_color = Color::srgba(0.0, 1.0, 0.4, 0.6);
+    let dest_color = STATE_SUCCESS.with_alpha(0.6);
 
     // Resolve the source pile from KeyboardDragState (when lifted) or
     // SelectionState (otherwise). Lifted takes precedence so the gold

solitaire_engine/src/ui_modal.rs

@@ -135,7 +135,8 @@ pub const MODAL_ENTER_START_SCALE: f32 = 0.96;
 /// Visual emphasis tier applied to a [`ModalButton`].
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ButtonVariant {
-    /// Loud yellow CTA — Confirm, Play Again. One per modal; right-aligned.
+    /// Cyan CTA (`ACCENT_PRIMARY`) — Confirm, Play Again, Resume. One per
+    /// modal; right-aligned in the actions row.
     Primary,
     /// Mid-emphasis — Cancel, Close, Done.
     Secondary,
@@ -332,14 +333,17 @@ pub fn spawn_modal_button<M: Component>(
     };
 
     let label_color = match variant {
-        // Primary buttons sit on the loud yellow accent — dark text on
-        // top reads well and passes AAA contrast.
+        // Primary buttons sit on the cyan accent — `BG_BASE` text on
+        // top reads well and passes AAA contrast against `#6fc2ef`.
         ButtonVariant::Primary => BG_BASE,
         ButtonVariant::Secondary | ButtonVariant::Tertiary => TEXT_PRIMARY,
     };
     let caption_color = match variant {
-        // Use a slightly muted version of the label colour so the chip
-        // reads as a secondary detail without disappearing.
+        // Muted near-black on the cyan 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.
         ButtonVariant::Primary => Color::srgba(0.0, 0.0, 0.0, 0.55),
         ButtonVariant::Secondary | ButtonVariant::Tertiary => TEXT_SECONDARY,
     };
@@ -395,9 +399,10 @@ fn hover_bg(variant: ButtonVariant) -> Color {
     }
 }
 
-/// Pressed-state background colour. Primary swaps to the magenta
-/// secondary accent for a moment of celebration; Secondary darkens to
-/// the base elevation; Tertiary darkens further.
+/// Pressed-state background colour. Primary swaps to the lavender
+/// secondary accent (`ACCENT_SECONDARY`, `#e1a3ee`) for a moment of
+/// celebration; Secondary darkens to the base elevation; Tertiary
+/// darkens further to `BG_ELEVATED_PRESSED`.
 fn pressed_bg(variant: ButtonVariant) -> Color {
     match variant {
         ButtonVariant::Primary => ACCENT_SECONDARY,