feat(engine): auto-save Time Attack sessions across launches

Classic, Zen, and Challenge already auto-saved correctly via the
existing game_state.json path — GameState carries mode and the
save/restore systems are mode-agnostic. Time Attack was the gap:
the per-deal GameState round-tripped fine, but the session-level
TimeAttackResource (10-minute countdown + accumulated wins)
defaulted on every launch, so closing mid-session reset the timer
and erased the win count.

Adds a sibling time_attack_session.json next to game_state.json,
atomic .tmp + rename via the existing save pattern. The new
TimeAttackSession struct carries remaining_secs, wins, and
saved_at_unix_secs (wall-clock anchor for stale-session detection).
load_time_attack_session_from_at takes an injectable now() so
tests can drive deterministic clock scenarios.

Load logic: if now_unix - saved_at_unix_secs > remaining_secs the
window expired in real time while the app was closed — return None
so the player isn't dropped into a session whose timer ran out
behind their back. Otherwise restore remaining_secs minus the
real-world elapsed delta. Handles clock-running-backwards (NTP
correction, VM clock drift) by clamping the elapsed delta at zero.

time_attack_plugin wires four new systems: load on Startup, clear
stale file when a fresh session starts (rare — only matters when
the previous session was abandoned + a new one started without
exit/relaunch), 30-second auto-save while a session is active,
delete file on natural expiry, and save on AppExit. The save file
is removed every time the session ends so a stale "session exists"
state can't pollute the next launch.

No GameState schema bump needed — the per-mode session lives in
its own file. stats / progress / achievements / settings unaffected.

8 new storage tests cover round-trip, expired-discard, time-decay,
atomic-write, missing-file, corrupt-file, delete idempotency, and
clock-backwards. 6 new plugin tests cover exit-persists,
exit-clears, auto-save-cadence, auto-save-noop-when-inactive,
new-session-clears-stale, and natural-expiry-clears.

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

solitaire_data/src/lib.rs

@@ -99,8 +99,11 @@ pub use stats::{StatsExt, StatsSnapshot};
 
 pub mod storage;
 pub use storage::{
-    cleanup_orphaned_tmp_files, delete_game_state_at, game_state_file_path, load_game_state_from,
-    load_stats, load_stats_from, save_game_state_to, save_stats, save_stats_to, stats_file_path,
+    cleanup_orphaned_tmp_files, delete_game_state_at, delete_time_attack_session_at,
+    game_state_file_path, load_game_state_from, load_stats, load_stats_from,
+    load_time_attack_session_from, load_time_attack_session_from_at, save_game_state_to,
+    save_stats, save_stats_to, save_time_attack_session_to, stats_file_path,
+    time_attack_session_path, time_attack_session_with_now, TimeAttackSession,
 };
 
 pub mod achievements;