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/onboarding_plugin.rs

@@ -313,6 +313,10 @@ fn complete_onboarding(
     despawn_screen(commands, screens);
     if let Some(s) = settings {
         s.0.first_run_complete = true;
+        // A fresh install has nothing "new" to announce — stamp the
+        // running release so the What's-new card (Phase I) only ever
+        // fires after an actual upgrade.
+        s.0.last_seen_whats_new = crate::whats_new_plugin::current_release_version();
         persist(path.map(|p| &p.0), &s.0);
     }
 }