chore: prune low-value tests per CLAUDE_SPEC.md §10 + WORKFLOW §8

The Quat-flagged "≥3 tests per feature" inflation produced 43 tests
that don't earn their existence — default-value, serde-derive
round-trips on plain structs, single-field clamp tests, near-
duplicates, and trivial constant-equals-itself tests. None pin a
behaviour contract or a regression on a real bug.

Removed across `solitaire_data` and `solitaire_core`:

  settings.rs   −22  default-value, round-trip, legacy-format,
                     and per-field sanitized clamp tests. Adjust
                     and load-error tests retained — those exercise
                     real method logic.
  progress.rs    −1  generic round-trip on plain struct.
  challenge.rs   −1  challenge_count() returns CHALLENGE_SEEDS.len()
                     literally — testing it asserts the implementation
                     against itself.
  game_state.rs  −3  undo_count starts at 0, GameMode default is
                     Classic, time_attack score starts at 0 — all
                     default-value tests on freshly-constructed state.
  card.rs        −5  rank_value_ace + rank_value_king subsumed by
                     rank_values_are_sequential; suit_red + suit_black
                     consolidated into one complementarity test;
                     card_face_up_field_reflects_construction was
                     testing the struct literal.

Workspace: 1208 → 1165 passing tests (−43). clippy --workspace
--all-targets clean.

Future work: brief sub-agents for tests that pin a behaviour
contract or regression on a real bug, not a count of N. See
`feedback_test_discipline.md` in auto-memory.

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

solitaire_core/src/game_state.rs

@@ -815,11 +815,6 @@ mod tests {
         assert!(g.undo_stack_len() <= 64);
     }
 
-    #[test]
-    fn undo_count_starts_at_zero() {
-        assert_eq!(new_game().undo_count, 0);
-    }
-
     #[test]
     fn undo_count_increments_on_each_undo() {
         let mut g = new_game();
@@ -900,11 +895,6 @@ mod tests {
         assert_eq!(g.score, 0);
     }
 
-    #[test]
-    fn zen_mode_default_is_classic_via_default_trait() {
-        assert_eq!(GameMode::default(), GameMode::Classic);
-    }
-
     #[test]
     fn zen_mode_field_persists_through_construction() {
         let g = GameState::new_with_mode(1, DrawMode::DrawThree, GameMode::Zen);
@@ -956,12 +946,6 @@ mod tests {
         assert!(g.undo().is_ok(), "undo must be permitted in TimeAttack mode");
     }
 
-    #[test]
-    fn time_attack_score_starts_at_zero() {
-        let g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::TimeAttack);
-        assert_eq!(g.score, 0);
-    }
-
     #[test]
     fn time_attack_draw_three_combination() {
         // TimeAttack + DrawThree is a valid combination; verify construction.