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/monologue.rs

@@ -819,12 +819,13 @@ mod tests {
         queue.push_anomaly(42, 0);
 
         // Pre-fill the monologue buffer (as if trigger_monologue already wrote)
-        let mut buffer = MonologueBuffer::default();
-        buffer.event = Some(MonologueEvent {
-            id: "existing_line".to_string(),
-            text: "I should keep this.".to_string(),
-            duration_seconds: 5.0,
-        });
+        let buffer = MonologueBuffer {
+            event: Some(MonologueEvent {
+                id: "existing_line".to_string(),
+                text: "I should keep this.".to_string(),
+                duration_seconds: 5.0,
+            }),
+        };
 
         world.spawn((
             PlayerCharacter,
@@ -1074,12 +1075,13 @@ mod tests {
         });
 
         // Pre-fill MonologueBuffer (e.g., from trigger_monologue)
-        let mut buffer = MonologueBuffer::default();
-        buffer.event = Some(MonologueEvent {
-            id: "existing_line".to_string(),
-            text: "Already have something to say.".to_string(),
-            duration_seconds: 5.0,
-        });
+        let buffer = MonologueBuffer {
+            event: Some(MonologueEvent {
+                id: "existing_line".to_string(),
+                text: "Already have something to say.".to_string(),
+                duration_seconds: 5.0,
+            }),
+        };
 
         world.spawn((
             PlayerCharacter,