feat(engine): right-click radial menu for quick-drop without dragging

Power-user shortcut: hold right-click on a face-up card, a small
ring of icons appears around the cursor with one entry per legal
destination, release over an icon to fire MoveRequestEvent. Skips
the drag motion entirely while preserving the existing
RightClickHighlight tint on the actual pile markers.

A new RadialMenuPlugin owns the flow. RightClickRadialState is a
two-state enum (Idle / Active) carrying the source pile, lifted
cards, pre-computed legal destinations + their world anchors, the
ring centre, and the currently hovered icon index. Four chained
systems handle press → cursor track → release/cancel → redraw, in
that order so a single-tick test can't observe a half-state.

Mutual exclusion with the left-button mouse drag is implicit —
RadialMenuPlugin only listens to MouseButton::Right while the
existing drag pipeline only listens to Left. RightClickHighlight
co-exists at a lower z (50) than the radial overlay (Z_RADIAL_MENU
= 60), so the brief pile-marker tint reads as the same set of legal
destinations the radial offers.

Cancel paths: release the right button outside any icon, press Esc,
or press the left button. All three reset state to Idle without
dispatching a move.

Visual: a centre dot at the press location plus N icons at radius
80 px around it. For one destination the icon sits at 12 o'clock;
for N icons they spread evenly clockwise. Hovered icon scales to
1.15× and tints STATE_SUCCESS so the focused choice is unambiguous.

Twelve new tests pin the contract — five system-level (open on
press over face-up card, release over destination fires move event,
release in dead space cancels, Esc cancels, face-down doesn't
open), seven on the pure helpers (radial_anchor_for_index,
radial_hovered_index, legal_destinations_for_card). Tests inject
cursor positions through a RadialCursorOverride resource so they
work under MinimalPlugins where there's no PrimaryWindow or Camera.

help_plugin's controls reference gains a new "Mouse" section
covering double-click auto-move, right-click highlight, and the
new "Hold RMB" radial. Onboarding slide 3 is intentionally left
keyboard-only — the radial is a power-user discovery, not a
first-run teach.

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

solitaire_app/src/main.rs

@@ -10,9 +10,9 @@ use solitaire_engine::{
     AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
     CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin, FontPlugin, GamePlugin, HelpPlugin,
     HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin, OnboardingPlugin, PausePlugin,
-    ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin, StatsPlugin,
-    SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin, UiFocusPlugin,
-    UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
+    ProfilePlugin, ProgressPlugin, RadialMenuPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin,
+    StatsPlugin, SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin,
+    UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
@@ -111,6 +111,7 @@ fn main() {
         .add_plugins(CardPlugin)
         .add_plugins(CursorPlugin)
         .add_plugins(InputPlugin)
+        .add_plugins(RadialMenuPlugin)
         .add_plugins(SelectionPlugin)
         .add_plugins(AnimationPlugin)
         .add_plugins(FeedbackAnimPlugin)