feat(engine): keyboard-only drag-and-drop via Tab → Enter → arrows → Enter

Players can now complete an entire game without a mouse. Tab cycles
the keyboard cursor across draggable card stacks, Enter "lifts" the
focused stack into a destination-pick mode, arrow keys (or Tab)
cycle through the legal targets only, and Enter confirms the move.
Esc cancels — single-press in Lifted reverts to source-pick keeping
focus, second-press clears the source selection entirely.

A new KeyboardDragState resource models the two-mode flow without
touching SelectionState's existing source-pick contract:

  Idle                         (Tab/Enter/auto-move via SelectionState)
  Lifted {
      source_pile, count, cards,
      legal_destinations,      pre-computed at lift time via
      destination_index,       can_place_on_foundation/_tableau
  }

Mutual exclusion with mouse drag is sentinel-based: keyboard lift
sets DragState.active_touch_id = u64::MAX (KEYBOARD_DRAG_TOUCH_ID),
existing mouse handlers in input_plugin already short-circuit when
active_touch_id is Some, and the cleanup path only clears DragState
when the sentinel is present so the mouse path is never stomped.
Conversely keyboard input is suppressed when a real mouse/touch
drag is active.

The visual lift reuses the existing drag z-lift and shadow path so
the keyboard-lifted stack reads the same as a mouse-lifted one;
update_selection_highlight gains a green destination indicator on
the focused legal target while Lifted.

help_plugin's canonical hotkey list grows a "Keyboard drag"
section (Tab/Enter/Arrows/Esc/Space) and onboarding slide 3 picks
up a "Tab → Enter" entry so first-run players see the full path.

Seven new headless tests pin the contract: Tab cycles to first
draggable pile, Enter lifts the stack, arrow keys cycle only legal
destinations, Enter with destination fires MoveRequestEvent and
clears state, Esc reverts to source-pick, mouse-drag-active
suppresses keyboard input, double-Esc clears source selection.

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

solitaire_engine/src/lib.rs

@@ -110,7 +110,9 @@ pub use settings_plugin::{
 };
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, HintCycleIndex, SettingsScrollPos, SyncStatus, SyncStatusResource};
-pub use selection_plugin::{SelectionHighlight, SelectionPlugin, SelectionState};
+pub use selection_plugin::{
+    KeyboardDragState, SelectionHighlight, SelectionPlugin, SelectionState,
+};
 pub use splash_plugin::{SplashAge, SplashPlugin, SplashRoot};
 pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};