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_core/src/game_state.rs

@@ -31,7 +31,8 @@ mod pile_map_serde {
     use crate::pile::{Pile, PileType};
 
     pub fn serialize<S: Serializer>(map: &HashMap<PileType, Pile>, s: S) -> Result<S::Ok, S::Error> {
-        let entries: Vec<(&PileType, &Pile)> = map.iter().collect();
+        let mut entries: Vec<(&PileType, &Pile)> = map.iter().collect();
+        entries.sort_by_key(|(k, _)| *k);
         entries.serialize(s)
     }
 
@@ -154,6 +155,7 @@ pub struct GameState {
     /// [`GAME_STATE_SCHEMA_VERSION`].
     #[serde(default = "schema_v1")]
     pub schema_version: u32,
+    #[serde(skip)]
     undo_stack: VecDeque<StateSnapshot>,
 }
 

solitaire_core/src/pile.rs

@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 use crate::card::{Card, Suit};
 
 /// Identifies which pile on the board a set of cards belongs to.
-#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
 pub enum PileType {
     /// The face-down draw pile.
     Stock,