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.

shared/protocol.ts

@@ -650,6 +650,63 @@ export interface DisastersOps {
   events: DisasterEvent[];
 }
 
+// --- Migration & refugees (v0.42.0, #64) --------------------------------------
+//
+// People moving between nations each turn: ordinary migration, refugee flows
+// (fleeing a war or disaster), a spreading diaspora, and an exodus (people
+// leaving with no destination). Each op names a `kind`, a source `from` (the
+// nation people leave), a destination `to` (required for every kind except
+// `exodus`), and a `scale` (small / moderate / large). The engine applies a
+// DETERMINISTIC effect chosen from the per-`scale` tables
+// `MIGRATION_SOURCE_EFFECTS` / `MIGRATION_DESTINATION_EFFECTS` (defined in
+// `server/abilities/migration.ts`) — no hidden randomness: the LLM chooses the
+// kind and scale, the tables decide the magnitude. The source LOSES a small
+// amount of population and economy, the destination GAINS a small amount of
+// population and economy (a `large` influx additionally strains the
+// destination's stability), both clamped to 0–100, mirrored to each side's
+// regions (`regionStats`), and each side's derived `stats` re-derived
+// afterwards (the same 0-region / no-entry guards as the hybrid / economy /
+// disasters layers). The from↔to relation moves one step toward the positive
+// end of the ally/neutral/rival/hostile ladder (`shiftRelationTowardPositive`)
+// — capped at `ally` for `refugees`, `neutral` for `migration` / `diaspora`,
+// and not at all for `exodus` — and a `war` relation is never touched. It is
+// **turn-transient** like the hybrid / disasters layers: there is NO `GameState`
+// field, no durable list, and no `db.ts` change — a continuing flow is simply
+// re-declared each turn it persists. It never moves territory (sole ownership
+// is untouched) and never creates or escalates a war.
+
+/** The kind of population flow a migration op describes (v0.42.0, #64). */
+export type MigrationKind = 'migration' | 'refugees' | 'diaspora' | 'exodus';
+
+/** The size of a migration flow (v0.42.0, #64). */
+export type MigrationScale = 'small' | 'moderate' | 'large';
+
+/**
+ * A single migration / refugee flow (v0.42.0, #64): the `kind`, the source
+ * `from` (a nation id / name — the nation people leave), the destination `to`
+ * (a nation id / name — required for every kind except `exodus`, which has no
+ * destination), the `scale`, and an optional short `detail`. The deterministic
+ * stat effects are looked up from `MIGRATION_SOURCE_EFFECTS` /
+ * `MIGRATION_DESTINATION_EFFECTS` by `scale`.
+ */
+export interface MigrationOp {
+  /** The kind of flow (one of {@link MigrationKind}). */
+  kind: MigrationKind;
+  /** The source nation (by id or name) people leave. */
+  from: string;
+  /** The destination nation (by id or name). Required for every kind except `exodus`. */
+  to?: string;
+  /** How many people moved (one of {@link MigrationScale}). */
+  scale: MigrationScale;
+  /** Optional short annotation (≤300 chars). */
+  detail?: string;
+}
+
+/** The migration block of a `TurnResult`. Required; default `{ ops: [] }`. */
+export interface MigrationOps {
+  ops: MigrationOp[];
+}
+
 /** The exact JSON object the world-simulator LLM must return for a turn. */
 export interface TurnResult {
   date: string;
@@ -819,6 +876,29 @@ export interface TurnResult {
    * `{ events: [] }`.
    */
   disasters: DisastersOps;
+  /**
+   * (v0.42.0, #64) Migration & refugee flows this turn — people moving between
+   * nations: ordinary `migration`, `refugees` fleeing a war or disaster, a
+   * spreading `diaspora`, and an `exodus` (people leaving with no destination).
+   * Each op names a `from` (the source nation, by id or name) and, for every
+   * kind except `exodus`, a `to` (the destination nation, by id or name), plus
+   * a `scale` (small / moderate / large). Applied directly by `applyTurnResult`
+   * (like the disasters / hybrid layers): the source LOSES a small amount of
+   * population and economy, the destination GAINS a small amount of population
+   * and economy (a `large` influx additionally strains the destination's
+   * stability by 2), both clamped to 0–100, mirrored to each side's regions,
+   * and each side's derived `stats` re-derived afterwards; the from↔to
+   * relation moves one step toward the positive end of the
+   * ally/neutral/rival/hostile ladder (capped at `ally` for `refugees`,
+   * `neutral` for `migration` / `diaspora`, not at all for `exodus`; a `war`
+   * relation is never touched). The layer is turn-transient (no `GameState`
+   * field, no `db.ts` change) — a continuing flow is simply re-declared each
+   * turn it persists. It never moves borders (sole ownership is untouched) and
+   * never creates or escalates a war. At most 1–2 per turn, each narrated in
+   * `events` (a `social` or `diplomatic` event). Required; default
+   * `{ ops: [] }`.
+   */
+  migration: MigrationOps;
   /**
    * (v0.27, #44) Map-marker ops this turn. `add` places new named markers
    * (the engine positions each at its region's centroid + a small