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
     // -----------------------------------------------------------------------