feat(simulation): #94 #99 active tier filtering and transition system

#94 — Active tier simulation (complete):
- Add ActiveSim marker to all 9 test world room NPC spawns
- Fix test entities in routine.rs and path_follow.rs to include ActiveSim
  so With<ActiveSim> queries match correctly in unit tests

#99 — Tier transition logic (complete):
- Implement update_tier_markers system in tier.rs
- Promotes/demotes tier markers by manhattan distance from PlayerCharacter:
  ≤40 tiles → ActiveSim, ≤120 → BackgroundSim, beyond → StateSaved
- Handles cross-z-level as u32::MAX (effectively unreachable)
- No-op when no PlayerCharacter entity present (headless tests safe)
- 11 new unit tests covering all distance bands and boundary cases
- TierPlugin now registers the system after movement::validate_movement

Also picks up extended test coverage added by hoshe:
- observer/tests.rs — 230 lines of perception observer tests
- sound.rs — additional sound event integration tests

All 548 tests pass.

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

server/src/simulation/path_follow.rs

@@ -98,6 +98,7 @@ mod tests {
         let entity = world
             .spawn((
                 Npc,
+                super::ActiveSim, // system requires With<ActiveSim> (#94)
                 TilePosition::new(0, 0, 0),
                 ComputedPath {
                     steps: vec![
@@ -129,6 +130,7 @@ mod tests {
         let entity = world
             .spawn((
                 Npc,
+                super::ActiveSim, // system requires With<ActiveSim> (#94)
                 TilePosition::new(2, 0, 0),
                 ComputedPath {
                     steps: vec![TilePosition::new(3, 0, 0)],
@@ -154,6 +156,7 @@ mod tests {
         let entity = world
             .spawn((
                 Npc,
+                super::ActiveSim, // system requires With<ActiveSim> (#94)
                 TilePosition::new(0, 0, 0),
                 ComputedPath {
                     steps: vec![TilePosition::new(1, 0, 0), TilePosition::new(2, 0, 0)],