feat(abilities): add migration & refugees lever (v0.42.0, #64)

Add the 10th modular LLM ability: a turn-transient TurnResult.migration
lever declaring population flows between nations (migration / refugees /
diaspora / exodus). The source loses a little population + economy, the
destination gains a little (a large influx also strains its stability),
and the from<->to relation is nudged one step toward the positive end of
the ally/neutral/rival/hostile ladder (capped by kind; a war is never
touched). Deterministic per-scale tables, no hidden randomness, clamped
0-100, mirrored to owned regions, re-derived aggregates; never moves
territory. Registered last in ABILITIES; user prompt unchanged
(user.txt golden byte-identical), system goldens re-baselined. Fixes #64.

server/engine.ts

@@ -94,6 +94,16 @@ export {
   DISASTER_TYPES,
   DISASTER_SEVERITIES,
   DISASTER_EFFECTS,
+  validateMigrationOps,
+  applyMigrationOp,
+  shiftRelationTowardPositive,
+  isMigrationKind,
+  isMigrationScale,
+  MIGRATION_KINDS,
+  MIGRATION_SCALES,
+  MIGRATION_SOURCE_EFFECTS,
+  MIGRATION_DESTINATION_EFFECTS,
+  LARGE_INFLUX_STABILITY_DIP,
 } from './abilities';
 
 // --- The LLM's getNeighbours tool (v0.21, #16 P2: contiguous region transfers) ---
@@ -622,6 +632,7 @@ export function validateTurnResult(
     leaders: { changes: [] },
     ideology: { changes: [] },
     disasters: { events: [] },
+    migration: { ops: [] },
     markers: { add: [], remove: [] },
   };