test+docs(simulation): address PR #159 review (T-1032)

- Hoshe: body_without_system_row test inserted a NULL system_id, impossible
  under production schema (system_id NOT NULL REFERENCES star_systems). Rewrite
  as body_with_orphan_system_id_* — non-NULL system_id with no matching
  star_systems row (the real case the LEFT JOIN guards) + NOT NULL in the test
  schema + corrected comment.
- Tyre: cascade.rs PERF/TODO comment said the region path 'defers to T-1032';
  T-1032 IS this PR, so production dispatch is now live. Update to track T-1028
  only and note the cost is live in production.

No production logic change. cargo test body_params_reader 7 pass, clippy clean.

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

server/src/atlas/body_params_reader.rs

@@ -286,8 +286,13 @@ mod tests {
     }
 
     #[test]
-    fn body_without_system_row_returns_null_stellar_fields() {
-        // Body has no system_id — LEFT JOIN produces NULL for star_systems columns.
+    fn body_with_orphan_system_id_returns_null_stellar_fields() {
+        // The LEFT JOIN guards against a body whose system_id has NO matching
+        // star_systems row (an incomplete DB — e.g. a body loaded before its
+        // system). It is NOT about a NULL system_id: production schema declares
+        // `bodies.system_id TEXT NOT NULL REFERENCES star_systems(system_id)`,
+        // so a NULL system_id can't exist. Mirror that NOT NULL here and test the
+        // real case — a non-NULL system_id pointing at an absent system row.
         let n = SEQ.fetch_add(1, Ordering::Relaxed);
         let path = std::env::temp_dir().join(format!("sr_bpr_ns_{}_{n}.db", std::process::id()));
         let _ = std::fs::remove_file(&path);
@@ -300,7 +305,7 @@ mod tests {
              );
              CREATE TABLE bodies (
                  body_id TEXT PRIMARY KEY,
-                 system_id TEXT,
+                 system_id TEXT NOT NULL,
                  hydrosphere TEXT,
                  atmosphere TEXT,
                  planet_class TEXT,
@@ -309,9 +314,9 @@ mod tests {
              );",
         )
         .expect("create tables");
-        // Insert body with no system_id (no system row exists).
+        // Body points at a system that doesn't exist in star_systems (orphan FK).
         conn.execute(
-            "INSERT INTO bodies (body_id, system_id, atmosphere) VALUES ('lonely', NULL, 'thin')",
+            "INSERT INTO bodies (body_id, system_id, atmosphere) VALUES ('lonely', 'GJ-ORPHAN', 'thin')",
             [],
         )
         .expect("insert");
@@ -320,7 +325,7 @@ mod tests {
         let reader = BodyParamsReader::open(&path).expect("open");
         let params = reader.read_body_params("lonely").expect("read");
         assert_eq!(params.atmosphere.as_deref(), Some("thin"));
-        // No system row → stellar fields NULL from LEFT JOIN.
+        // No matching star_systems row → stellar fields NULL from the LEFT JOIN.
         assert!(params.spectral_class.is_none());
         assert!(params.star_type.is_none());
     }