feat(engine): convert PauseScreen to modal + add ForfeitConfirmScreen

Pause overlay drops its bespoke full-screen layout and is rebuilt on
the standard `ui_modal` scaffold: uniform scrim, centred card, real
Resume (Primary, Esc) and Forfeit (Tertiary, G) action buttons. The
Draw Mode row stays inline in the body so the existing toggle still
fires `SettingsChangedEvent`.

The G-key double-press toast countdown is replaced with a real
modal: `G` (or clicking Forfeit on Pause) fires the new
`ForfeitRequestEvent`, which `PausePlugin` answers by spawning
`ForfeitConfirmScreen` at `Z_PAUSE_DIALOG` (above pause). The modal
exposes Cancel + "Yes, forfeit" buttons plus Y/Enter/N/Esc
accelerators; confirmation despawns both modals, clears
`PausedResource`, and fires `ForfeitEvent` for `StatsPlugin`.

`toggle_pause` now early-returns when a forfeit modal is visible (and
runs `.before(handle_forfeit_keyboard)`) so an Esc that closes the
forfeit modal doesn't also re-open pause in the same frame.

The legacy `forfeit_countdown` field, `FORFEIT_CONFIRM_WINDOW`
constant, and the six pure-function countdown tests are removed; new
tests cover the modal-spawn / confirm / cancel paths and the active-
game predicate that still gates the G hotkey.

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

solitaire_engine/src/lib.rs

@@ -69,9 +69,9 @@ pub use font_plugin::{FontPlugin, FontResource};
 pub use cursor_plugin::CursorPlugin;
 pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
-    ForfeitEvent, GameWonEvent, HelpRequestEvent, HintVisualEvent, InfoToastEvent,
-    ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent,
-    NewGameRequestEvent, PauseRequestEvent, StartChallengeRequestEvent,
+    ForfeitEvent, ForfeitRequestEvent, GameWonEvent, HelpRequestEvent, HintVisualEvent,
+    InfoToastEvent, ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent,
+    NewGameConfirmEvent, NewGameRequestEvent, PauseRequestEvent, StartChallengeRequestEvent,
     StartDailyChallengeRequestEvent, StartTimeAttackRequestEvent, StartZenRequestEvent,
     StateChangedEvent, SyncCompleteEvent, ToggleAchievementsRequestEvent,
     ToggleLeaderboardRequestEvent, ToggleProfileRequestEvent, ToggleSettingsRequestEvent,
@@ -87,7 +87,7 @@ pub use hud_plugin::{
 pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, LeaderboardScreen};
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};
-pub use pause_plugin::{PausePlugin, PauseScreen, PausedResource};
+pub use pause_plugin::{ForfeitConfirmScreen, PausePlugin, PauseScreen, PausedResource};
 pub use profile_plugin::{ProfilePlugin, ProfileScreen};
 pub use settings_plugin::{
     SettingsChangedEvent, SettingsPlugin, SettingsResource, SettingsScreen, SFX_STEP,