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_data/src/settings.rs

@@ -257,6 +257,14 @@ pub struct Settings {
     /// deserialize cleanly to `GameMode::Classic` via `#[serde(default)]`.
     #[serde(default)]
     pub last_mode: GameMode,
+    /// The release version whose "What's new" card the player has already
+    /// seen (e.g. `"0.46.0"`). Empty on installs that predate the card,
+    /// which correctly reads as "there is news to show" after an upgrade;
+    /// fresh installs stamp it silently when onboarding completes. Older
+    /// `settings.json` files deserialize cleanly to `""` via
+    /// `#[serde(default)]`.
+    #[serde(default)]
+    pub last_seen_whats_new: String,
     /// Custom public name displayed on the leaderboard. When `None`, the
     /// player's server `username` is used instead. Trimmed to 32 characters
     /// before submission. Older `settings.json` files written before this
@@ -425,6 +433,7 @@ impl Default for Settings {
             replay_move_interval_secs: default_replay_move_interval_secs(),
             last_difficulty: None,
             last_mode: GameMode::Classic,
+            last_seen_whats_new: String::new(),
             leaderboard_display_name: None,
             leaderboard_opted_in: false,
             take_from_foundation: true,