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/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