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/lib.rs

@@ -64,6 +64,7 @@ pub mod ui_modal;
 pub mod ui_theme;
 pub mod ui_tooltip;
 pub mod weekly_goals_plugin;
+pub mod whats_new_plugin;
 pub mod win_summary_plugin;
 pub mod you_hub_plugin;
 
@@ -194,6 +195,7 @@ pub use ui_modal::{
 };
 pub use ui_tooltip::{Tooltip, UiTooltipPlugin};
 pub use weekly_goals_plugin::{WeeklyGoalCompletedEvent, WeeklyGoalsPlugin};
+pub use whats_new_plugin::{WhatsNewPending, WhatsNewPlugin, WhatsNewScreen};
 pub use win_summary_plugin::{
     ScreenShakeResource, SessionAchievements, WinSummaryPending, WinSummaryPlugin, format_win_time,
 };