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/npc/disclosure.rs

@@ -708,8 +708,10 @@ mod tests {
         )]);
 
         // Set computed_tick = 1 (non-zero). Tick 5 is within the 30-tick refresh window.
-        let mut candidates = DisclosureCandidates::default();
-        candidates.computed_tick = 1;
+        let candidates = DisclosureCandidates {
+            computed_tick: 1,
+            ..Default::default()
+        };
 
         let npc = app
             .world_mut()

server/src/npc/mood.rs

@@ -656,7 +656,7 @@ mod tests {
         // from derive_mood(). They must be set externally by other systems
         // (e.g., observation pipeline for Suspicious, activity scheduler for Focused).
         // This test documents the invariant: derive_mood never emits these states.
-        use std::collections::HashSet;
+        use std::collections::BTreeSet;
 
         let phases = [
             DayPhase::Morning,
@@ -668,7 +668,7 @@ mod tests {
         let thresholds: &[i16] = &[0, 1, 50, 100];
         let warm_flags = [false, true];
 
-        let mut observed = HashSet::new();
+        let mut observed = BTreeSet::new();
         for &phase in &phases {
             for &stress in stresses {
                 for &threshold in thresholds {

server/src/npc/vision.rs

@@ -358,10 +358,12 @@ mod tests {
         let target_sid = registry.register(target);
         spatial.update(target, pos(16, 14));
 
-        // Wall between NPC and target
-        let mut walkability = world.resource_mut::<WalkabilityMap>();
-        walkability.set_walkable(&pos(16, 15), false);
-        drop(walkability);
+        // Wall between NPC and target — scope the mutable resource borrow so it
+        // is released before the world.insert_resource calls below.
+        {
+            let mut walkability = world.resource_mut::<WalkabilityMap>();
+            walkability.set_walkable(&pos(16, 15), false);
+        }
 
         world.insert_resource(registry);
         world.insert_resource(spatial);