fix(simulation): decorrelate per-footprint tag fields + test gaps (#957)

PR #153 review (Hoshe H1): zone_type/era/floors/flavor all shared one fp_chain
and each took splitmix64(seed) of the same value, correlating the fields (era
tracking floor height across a city). Derive a distinct sub-chain per field so
they draw independent entropy. Block_tags aren't serialized, so no fixture
change; behavior stays deterministic.

Also (review): n=1 corridor test (empty, no panic), Frontier-density waterfront
test, and a comment on the reserved `_morphology` param (Tyre).

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

server/src/atlas/skeleton_gen.rs

@@ -798,6 +798,9 @@ fn block_on_quarter_edge(row: u8, col: u8, edge: Edge) -> bool {
 fn subdivide_block_footprints(
     density_pct: u8,
     bulk: &BulkClass,
+    // Reserved for morphology-specific lot shaping (narrow canyon lots,
+    // pier-oriented delta lots) once that detail is designed; the waterfront
+    // rule already consumes the water-facing geometry via `waterfront`.
     _morphology: &MorphologyZone,
     waterfront: Option<Edge>,
     seed: SeedChain,
@@ -877,17 +880,31 @@ fn assign_block_tags(
         .into_iter()
         .enumerate()
         .map(|(i, footprint)| {
+            // Distinct per-field sub-chains so zone / era / floors / flavor draw
+            // independent entropy — sharing one `fp_chain` made every field a
+            // modulo of the same `splitmix64(seed)` value and correlated them
+            // (e.g. construction era tracking floor height across the city).
             let fp_chain = block_chain.derive(SeedDomain::Block, i as u64 + 1);
-            let zone_type_id = zone_type_for(&block.zoning, economic_role, setting, fp_chain);
+            let zone_type_id = zone_type_for(
+                &block.zoning,
+                economic_role,
+                setting,
+                fp_chain.derive(SeedDomain::Block, 0),
+            );
             let entry_class =
                 building_entry_class(&block.zoning, &skeleton.layout_mode, prosperity);
-            let (era, era_cause) = construction_era(founding_age_years, prosperity, fp_chain);
+            let (era, era_cause) = construction_era(
+                founding_age_years,
+                prosperity,
+                fp_chain.derive(SeedDomain::Block, 1),
+            );
             let initial = initial_condition(prosperity, &era_cause);
-            let extent = floor_extent(block.density_pct, fp_chain);
+            let extent = floor_extent(block.density_pct, fp_chain.derive(SeedDomain::Block, 2));
             // D-232: deterministic (seed + zone_type) → flavor index into the
             // body's K-tag trait selection.
+            let flavor_seed = fp_chain.derive(SeedDomain::Block, 3).seed();
             let flavor_index =
-                (splitmix64(fp_chain.seed() ^ zone_type_hash(&zone_type_id)) % flavor_n) as u8;
+                (splitmix64(flavor_seed ^ zone_type_hash(&zone_type_id)) % flavor_n) as u8;
             BuildingPropertyTag {
                 zone_type_id,
                 footprint,
@@ -1724,6 +1741,13 @@ mod tests {
         assert_eq!(derive_corridors(&mesh, &MorphologyZone::Delta).len(), 3); // hub-spoke
     }
 
+    #[test]
+    fn corridors_single_node_returns_empty() {
+        let aps = derive_access_points(&[], &[]);
+        assert_eq!(aps.len(), 1);
+        assert!(derive_corridors(&aps, &MorphologyZone::AlluvialPlain).is_empty());
+    }
+
     #[test]
     fn hub_spoke_shares_a_common_node() {
         let aps = derive_access_points(&[0, 2, 4, 6], &[]);
@@ -1769,6 +1793,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn waterfront_applies_at_frontier_density() {
+        // A coastal frontier settlement (few large lots) still presents to the quay.
+        let inland = subdivide_block_footprints(
+            10,
+            &BulkClass::BulkSolid,
+            &MorphologyZone::CoastalLowland,
+            None,
+            SeedChain::root(11),
+        );
+        let quay = subdivide_block_footprints(
+            10,
+            &BulkClass::BulkSolid,
+            &MorphologyZone::CoastalLowland,
+            Some(Edge::North),
+            SeedChain::root(11),
+        );
+        let min_y = |v: &[TileRect]| v.iter().map(|r| r.origin.1).min().unwrap_or(u8::MAX);
+        assert!(min_y(&quay) < min_y(&inland));
+    }
+
     #[test]
     fn skeleton_has_streets_and_local_lattice() {
         let ctx = make_context(PoliticalArchetype::Commission, WorldTier::Regional);