feat(engine): one-shot What's-new card after updates (Phase I)

ObtainX updates install silently, so shipped features went unnoticed.
On the first launch where the running release differs from the new
Settings::last_seen_whats_new, a dismissible card summarises the
latest CHANGELOG.md section (embedded; its top version doubles as the
app's release identity — no build-time version plumbing). Internal
sections are dropped and bullets reduce to their bold lead sentence.

Launch beat: splash -> onboarding (first run) -> what's-new -> Home;
spawn_home_on_launch waits on the new WhatsNewPending resource. Fresh
installs never see the card — onboarding completion stamps the current
version silently. The seen-stamp persists on spawn, not dismissal, so
the card can never nag twice.

8 new tests (changelog parsing incl. the real embedded file, upgrade/
seen/fresh-install gating, dismissal). Workspace + clippy green.

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

solitaire_engine/src/core_game_plugin.rs

@@ -21,7 +21,7 @@ use crate::{
     SafeAreaInsetsPlugin, SelectionPlugin, SettingsPlugin, SolutionPlaybackPlugin, SplashPlugin,
     StatsPlugin, SyncProvider, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin,
     TouchSelectionPlugin, UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin,
-    WinSummaryPlugin,
+    WhatsNewPlugin, WinSummaryPlugin,
 };
 #[cfg(not(target_arch = "wasm32"))]
 use crate::{
@@ -115,6 +115,7 @@ impl Plugin for CoreGamePlugin {
             .add_plugins(PausePlugin)
             .add_plugins(SettingsPlugin::default())
             .add_plugins(OnboardingPlugin)
+            .add_plugins(WhatsNewPlugin)
             .add_plugins(WinSummaryPlugin)
             .add_plugins(UiModalPlugin)
             .add_plugins(UiFocusPlugin)