feat(engine): Phase F touch action bar — thumb-reach Undo/Draw/Hint + hold-to-repeat undo

Touch layout (USE_TOUCH_UI_LAYOUT) restructures the bottom action bar
to five buttons: an enlarged Undo / Draw / Hint trio (96x64px targets,
1.35x labels) between compact Menu and Pause. Draw is new — it fires
the same DrawRequestEvent as tapping the stock, so the most frequent
action no longer needs a reach to the top of a tall folded screen.
Help, Modes, and New Game leave the touch bar (they live in Menu ->
System, the Home grid, and Home's hero respectively). Desktop keeps
the seven-button bar unchanged (decision 5: touch-only).

Holding Undo now steps back repeatedly after a 0.45s delay (5.5/s),
each step through the normal request path so the scoring penalty
applies. New self-ambiguous DrawRequestWriters set keeps the ambiguity
gate at zero with the fourth DrawRequestEvent writer.

6 new hud_plugin tests; workspace suite + clippy green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/game_plugin/mod.rs

@@ -86,6 +86,14 @@ pub struct NewGameRequestWriters;
 #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
 pub struct UndoRequestWriters;
 
+/// Self-ambiguous set for writers of `DrawRequestEvent` — same rationale as
+/// [`NewGameRequestWriters`]: consumers drain the queue, append order is
+/// meaningless (#143). Members: keyboard `D`/`Space`, stock click, touch
+/// stock tap (input_plugin), and the touch action bar's Draw button
+/// (hud_plugin, Phase F).
+#[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
+pub struct DrawRequestWriters;
+
 /// Self-ambiguous set for writers of `InfoToastEvent` — toasts queue in
 /// arrival order and any same-frame order is fine (#143).
 #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]