fix(simulation): address PR #46 review — zone tests, doc accuracy, TBD comment

Add 3 zone enrichment tests to observer pipeline (tile inside zone,
tile outside zone, absent ZoneMap resource). Fix misleading Decay
Observatory doc comment that claimed Direct start when test starts at
KnowsDetails. Add production population TBD note on ZoneMap.

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

server/src/simulation/zone.rs

@@ -18,6 +18,9 @@ use bevy_ecs::prelude::*;
 ///
 /// Zone IDs are opaque u16 values — the client maintains its own
 /// `zone_id → zone_name / temperature_tint / ambient_layer` mapping.
+///
+/// Production population path is TBD — currently populated only by
+/// Gauntlet room builders via `set_rect` / `set`.
 #[derive(Resource, Debug, Default)]
 pub struct ZoneMap {
     zones: BTreeMap<(i32, i32, i32), u16>,