feat(core): add pile, error, deck, rules, scoring modules with tests

Implements PileType/Pile, MoveError (thiserror), Deck with seeded shuffle,
deal_klondike layout, foundation/tableau placement rules, and Windows XP
Standard scoring — 41 tests, clippy clean.

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

solitaire_core/src/error.rs

@@ -0,0 +1,36 @@
+use thiserror::Error;
+
+/// All reasons a game move can be rejected.
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
+pub enum MoveError {
+    #[error("invalid source pile")]
+    InvalidSource,
+    #[error("invalid destination pile")]
+    InvalidDestination,
+    #[error("source pile is empty")]
+    EmptySource,
+    #[error("move violates rules: {0}")]
+    RuleViolation(String),
+    #[error("undo stack is empty")]
+    UndoStackEmpty,
+    #[error("game is already won")]
+    GameAlreadyWon,
+    #[error("stock and waste are both empty")]
+    StockEmpty,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rule_violation_includes_message() {
+        let e = MoveError::RuleViolation("king only on empty".into());
+        assert!(e.to_string().contains("king only on empty"));
+    }
+
+    #[test]
+    fn undo_stack_empty_has_non_empty_message() {
+        assert!(!MoveError::UndoStackEmpty.to_string().is_empty());
+    }
+}