fix(simulation): address PR #168 review — fallback diagnostic, parity coverage, doc (T-1039)

Hoshe H1: the morphology_zone AlluvialPlain fallback fired silently. Now logs
tracing::debug! when the district grid is empty (expected: params-missing body /
early cascade) and tracing::warn! when a populated grid misses the key (indicates a
pixel->district convention bug) — no more silently-wrong topology.

Hoshe H2: the arrangement_pattern parity tripwire only covered transit_hub with
Commission + Corporate. Added Pioneer/Industrial/Military/Academic rows so an
accidental archetype-conditionalization of the cross-archetype override is caught.

Tyre (non-blocking): clarified the build_skeleton_work_item doc — the
arrangement_pattern re-derivation call lives in generate_quarter_skeleton (the
consumer), not in this function.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/plugin.rs

@@ -201,7 +201,9 @@ fn drain_generation_completions(
 /// `arrangement_pattern` is **re-derived** at L4 from `(political_archetype,
 /// economic_role)` via the same pure function used at L3 (T-1039 OPTION (b) —
 /// locked, no `CityGenerationContext` field added). Re-derivation is provably
-/// identical to the L3 value (pure total function, no RNG).
+/// identical to the L3 value (pure total function, no RNG). The re-derivation call
+/// itself lives in the consumer (`generate_quarter_skeleton`), not in this function
+/// — `build_skeleton_work_item` only threads the inputs it needs.
 ///
 /// `quarter_id` is the canonical D-194/D-230 derivation from `(world_seed, body,
 /// city)` — not the `city_id * 10` placeholder.
@@ -235,10 +237,29 @@ fn build_skeleton_work_item(
     // Convert the placement's working-grid pixel position to a DistrictPos using
     // the canonical scale constant — no hardcoded magic numbers here.
     let district_pos = scale::heightmap_pixel_to_district(placement.position);
-    context.morphology_zone = districts
-        .get(&district_pos)
-        .map(|d| d.morphology_zone)
-        .unwrap_or(MorphologyZone::AlluvialPlain);
+    context.morphology_zone = match districts.get(&district_pos) {
+        Some(d) => d.morphology_zone,
+        None => {
+            // An empty grid is the expected params-missing / early-cascade case
+            // (debug); a miss against a *populated* grid means the pixel→DistrictPos
+            // conversion is off — a real bug worth a warning, not a silent wrong
+            // topology.
+            if districts.is_empty() {
+                tracing::debug!(
+                    city_id = placement.city_id,
+                    ?district_pos,
+                    "morphology_zone fallback to AlluvialPlain: district grid not built for this body"
+                );
+            } else {
+                tracing::warn!(
+                    city_id = placement.city_id,
+                    ?district_pos,
+                    "morphology_zone fallback to AlluvialPlain: pos not in populated district grid — check pixel→district convention"
+                );
+            }
+            MorphologyZone::AlluvialPlain
+        }
+    };
 
     // ── T-1043: road_entry_directions from road_graph ───────────────────────────
     // Find this city's settlement node index in the road graph (O(n) scan on a
@@ -769,6 +790,29 @@ mod tests {
                 "transit_hub",
                 ArrangementPattern::HubAndSpoke,
             ),
+            // transit_hub must override EVERY archetype (the guard fires before the
+            // archetype match) — cover the rest so an accidental
+            // archetype-conditionalization of the override can't slip through.
+            (
+                PoliticalArchetype::Pioneer,
+                "transit_hub",
+                ArrangementPattern::HubAndSpoke,
+            ),
+            (
+                PoliticalArchetype::Industrial,
+                "transit_hub",
+                ArrangementPattern::HubAndSpoke,
+            ),
+            (
+                PoliticalArchetype::Military,
+                "transit_hub",
+                ArrangementPattern::HubAndSpoke,
+            ),
+            (
+                PoliticalArchetype::Academic,
+                "transit_hub",
+                ArrangementPattern::HubAndSpoke,
+            ),
         ];
 
         for (archetype, role, expected_pattern) in cases {