fix(core,sync,data): deterministic pile serialization, undo skip, url-encode bytes, merge_at

- Derive PartialOrd+Ord on PileType and sort pile entries in pile_map_serde
  before serializing so save-file output is deterministic (M-4)
- Add #[serde(skip)] to undo_stack so transient undo history is never written
  to save files, eliminating unnecessary bloat (M-3)
- Add merge_at() accepting an explicit resolved_at timestamp so callers can
  inject the server-side time; merge() wraps it with Utc::now() for
  backwards compatibility (M-1)
- Fix url_encode to percent-encode UTF-8 bytes rather than Unicode codepoints
  so multi-byte characters produce RFC 3986-compliant output (M-2)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_sync/src/lib.rs

@@ -12,7 +12,7 @@ pub mod progress;
 pub mod stats;
 
 pub use achievements::AchievementRecord;
-pub use merge::merge;
+pub use merge::{merge, merge_at};
 pub use progress::{level_for_xp, PlayerProgress};
 pub use stats::StatsSnapshot;