fix(simulation): PR #178 review round — H1-H4, T1, T2

H1: split_edge_at now splits length_cells geometric-distance proportionally (fixed-order f64 polyline sums, halves sum exactly to parent) — subsumes the 2-point-parent all-or-nothing bug. H2: chained-snap test (minor onto minor spur) + determinism assertion actually exercising attach_minors + direct 2-point split unit test. H3: cascade-level RailHeadFacing end-to-end assertion on a plus-shaped landmass fixture (2 deterministic degree-3 settlement junctions). H4+T2: STANDALONE_HQ_JOIN_SQL shared constant, both readers compose it; believability lockstep unit test. T1: HUB_SPACING_DIAG_PX doc cross-refs D-243 elastic seam.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/city_context_reader.rs

@@ -62,6 +62,25 @@ use crate::simulation::generator::{
     ProductionUbiquity, SettingType, SettlementClass, WorldTier,
 };
 
+// ---------------------------------------------------------------------------
+// Shared SQL fragments
+// ---------------------------------------------------------------------------
+
+/// The D-242 standalone-corp-HQ LEFT JOIN (T-1076 §1; single source of truth,
+/// PR #178 T2). Marks an `atlas_city_names` row (aliased `acn`) as a Standalone
+/// corp-HQ company town when a `corporations` row (aliased `c`) matches on
+/// `hq_placement = 'Standalone'`, `headquarters_body = body_id`, and
+/// `proper_name = name` — the exact shape
+/// `populate_standalone_hq_settlements` (economy_import/corporations.py) emits
+/// settlement rows with. Composed into a query whose FROM clause aliases
+/// `atlas_city_names AS acn`; the flag is read as `c.corp_id IS NOT NULL`.
+/// Used by [`CityContextReader::read_body_settlements`] and the believability
+/// harness's `read_cities` — extend both if the join shape ever changes.
+pub(crate) const STANDALONE_HQ_JOIN_SQL: &str = "LEFT JOIN corporations AS c
+        ON c.hq_placement = 'Standalone'
+       AND c.headquarters_body = acn.body_id
+       AND c.proper_name = acn.name";
+
 // ---------------------------------------------------------------------------
 // Error type
 // ---------------------------------------------------------------------------
@@ -266,14 +285,12 @@ impl CityContextReader {
     /// any malformed field), this method is best-effort: it never fails on row
     /// content, only on a DB/connection error.
     ///
-    /// `is_standalone_hq` (T-1076 §1): a LEFT JOIN against `corporations`
-    /// marks the rows that are D-242 Standalone corp-HQ company towns
-    /// (`hq_placement = 'Standalone'`, `headquarters_body = body_id`,
-    /// `proper_name = name` — the exact shape `populate_standalone_hq_settlements`
-    /// emits them with). The road-graph hub rule demotes these to minor nodes
-    /// regardless of population. The believability harness's own settlement
-    /// read (`believability::read_cities`) performs the identical join — keep
-    /// the two in lockstep.
+    /// `is_standalone_hq` (T-1076 §1): the [`STANDALONE_HQ_JOIN_SQL`] LEFT JOIN
+    /// marks the rows that are D-242 Standalone corp-HQ company towns. The
+    /// road-graph hub rule demotes these to minor nodes regardless of
+    /// population. The believability harness's own settlement read
+    /// (`believability::read_cities`) composes the same shared constant —
+    /// single source of truth for the join (PR #178 T2).
     pub fn read_body_settlements(
         &self,
         body_id: &str,
@@ -282,19 +299,17 @@ impl CityContextReader {
             .conn
             .lock()
             .map_err(|e| CityContextReadError::Db(format!("mutex poisoned: {e}")))?;
+        let sql = format!(
+            "SELECT acn.id, acn.name, acn.economic_role, acn.population,
+                    acn.settlement_class, COALESCE(acn.kind, 'city'),
+                    c.corp_id IS NOT NULL
+             FROM atlas_city_names AS acn
+             {STANDALONE_HQ_JOIN_SQL}
+             WHERE acn.body_id = ?1
+             ORDER BY acn.id"
+        );
         let mut stmt = conn
-            .prepare(
-                "SELECT acn.id, acn.name, acn.economic_role, acn.population,
-                        acn.settlement_class, COALESCE(acn.kind, 'city'),
-                        c.corp_id IS NOT NULL
-                 FROM atlas_city_names AS acn
-                 LEFT JOIN corporations AS c
-                        ON c.hq_placement = 'Standalone'
-                       AND c.headquarters_body = acn.body_id
-                       AND c.proper_name = acn.name
-                 WHERE acn.body_id = ?1
-                 ORDER BY acn.id",
-            )
+            .prepare(&sql)
             .map_err(|e| CityContextReadError::Db(e.to_string()))?;
         let rows = stmt
             .query_map([body_id], |row| {