style(server): clear clippy-1.93 cfg(test)/test-target debt (#967)

Manual clippy-1.93 fixes that the prior machine-applicable sweep couldn't auto-
apply, all in cfg(test) modules and tests/ targets (invisible to the lib-only
pre-push clippy, hence accumulated unflagged):

- disallowed_types HashSet/HashMap → BTreeSet/BTreeMap (determinism rule):
  shadowcast_bench.rs (×8, (i32,i32) keys), mood.rs, sound.rs. SoundEventKind
  gains a PartialOrd/Ord derive (fieldless Copy enum) so it is BTree-usable.
- field_reassign_with_default → struct-init: disclosure.rs, monologue.rs (×2),
  save_io.rs (keeps `mut` for the deliberate last-write-wins overwrite).
- assertions_on_constants on the EAVESDROP_THRESHOLD invariant → compile-time
  `const _: () = assert!(...)`: listening.rs, cross_room_transitions.rs. This is
  stronger than the runtime assert and needs no #[allow].
- approx_constant: settings/types.rs round-trip literal 3.14 → 2.5 (the value is
  arbitrary test data, never meant to be PI — change avoids both the lint and a
  suppression).
- drop_non_drop: vision.rs early Mut<WalkabilityMap> release → scoped block.
- unnecessary_get_then_check → contains_key: information_boundaries.rs (×3).
- cloned_ref_to_slice_refs → std::slice::from_ref: triangle_validation.rs.
- unused_must_use: input.rs dropped the unused .id() on a spawn.

cargo clippy --all-targets -- -D warnings is clean; cargo test green.

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

server/src/simulation/save_io.rs

@@ -756,11 +756,11 @@ mod tests {
     /// The warn! in process_player_input fires; here we just confirm last-write-wins.
     #[test]
     fn pending_command_overwrite_last_write_wins() {
-        let mut pending = SaveLoadPending::default();
-
-        pending.pending = Some(SaveLoadCommand::Save {
-            path: PathBuf::from("/tmp/first.msgpack"),
-        });
+        let mut pending = SaveLoadPending {
+            pending: Some(SaveLoadCommand::Save {
+                path: PathBuf::from("/tmp/first.msgpack"),
+            }),
+        };
         // Overwrite with a Load command
         pending.pending = Some(SaveLoadCommand::Load {
             path: PathBuf::from("/tmp/second.msgpack"),