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.

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 All notable changes to **History** are listed here, newest first.
 
+## v0.42.0 — 2026-10-16 10:12
+- **#64: Migration & refugees — a `TurnResult.migration` lever letting the LLM declare population flows between nations (ordinary migration / refugee flows / a spreading diaspora / an exodus), with a deterministic per-`scale` effect: the source loses a little population and 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 ladder.** It is a **turn-transient** feature, exactly like the v0.34 `hybrid` and v0.41 `disasters` layers: **no new `GameState` field, no `db.ts` change, no durable list** — the ops are consumed inside `applyTurnResult` and leave nothing behind. It lands as the **10th ability** of the v0.40.0 modular system, in a new `server/abilities/migration.ts` module, **registered last** in the `ABILITIES` registry (after `disasters`).
+  - **Types (`shared/protocol.ts`).** New `MigrationKind` (`'migration' | 'refugees' | 'diaspora' | 'exodus'`), `MigrationScale` (`'small' | 'moderate' | 'large'`), `MigrationOp` (`{ kind; from; to?; scale; detail? }` — `to` is required for every kind except `exodus`, which has no destination), and `MigrationOps` (`{ ops: MigrationOp[] }`); `TurnResult.migration` is **required**, placed between `disasters` and `markers`, default `{ ops: [] }`.
+  - **Ability module (`server/abilities/migration.ts`).** `field: 'migration'`; `default()` / `stub()` → a **fresh** `{ ops: [] }` each call; `validate` wraps the exported `validateMigrationOps` (defensive, never throws — accepts the `{ ops: [...] }` object or a bare array; each op needs a valid `kind`, a valid `scale`, a non-empty `from` (trimmed, ≤200), and — for every kind except `exodus` — a non-empty `to` (trimmed, ≤200); an optional `detail` (trimmed, ≤300, empty dropped) — anything else is dropped); `apply` runs `applyMigrationOp` per op; `schemaLine()` / `constraint()` supply the prompt text; `payload()` → `{}` (no user-payload keys — both refs resolve from the existing nation ref table); `isMeaningful` = `ops.length > 0`; **no** `stateFields` (turn-transient, no durable `GameState` field).
+  - **Deterministic effects (tables, no hidden randomness).** `MIGRATION_SOURCE_EFFECTS` per scale — the source LOSES: small `{population:-1, economy:-1}`, moderate `{population:-2, economy:-2}`, large `{population:-4, economy:-3}`. `MIGRATION_DESTINATION_EFFECTS` per scale — the destination GAINS: small `{population:+1, economy:+1}`, moderate `{population:+2, economy:+2}`, large `{population:+3, economy:+3}`. `LARGE_INFLUX_STABILITY_DIP = -2` — a `large` influx additionally strains the destination's stability by 2. All clamped to 0–100.
+  - **Apply semantics (`applyMigrationOp`).** Resolution is drop-first: `from` must resolve to a nation (id first, then a unique name; unknown / ambiguous → drop); for every kind except `exodus` the `to` must resolve to a nation **distinct from** `from` (missing / unknown / ambiguous / self-flow → the **whole op** is dropped, the source untouched). Only then is the source dip applied (all four kinds, including `exodus`); `exodus` then returns (no destination, no nudge). Otherwise the destination gain is applied (plus the `large` stability strain), and the from↔to relation is nudged by the self-contained `shiftRelationTowardPositive` — the mirror of the hybrid layer's `shiftRelationTowardHostile`, but toward `ally` (`POSITIVE_ORDER = ['ally','neutral','rival','hostile']`, cap `ally` for `refugees`, `neutral` for `migration` / `diaspora`, none for `exodus`; a missing relation starts at `neutral`; a `war` relation is **never** touched — the nudge neither fabricates nor escalates a war). Each affected nation's dip is clamped to 0–100, **mirrored to every region it owns** (`regionStats`), and its derived `stats` **re-derived** afterwards (the same 0-region / no-entry guards as hybrid / economy / disasters — a 0-region nation keeps its directly-mutated value and the effect persists). It **never moves territory** (sole ownership is untouched). The **stub never** proposes a migration (`{ ops: [] }`), so e2e (CI, `LLM_STUB=1`) is unaffected and the path is covered by unit tests only.
+  - **Registry & prompt.** `migration` is added **last** in `ABILITIES` (existing order untouched); the system prompt gains the `migration` schema line and its constraint bullet ("at most 1–2 per turn, each narrated in `events` as a `social` or `diplomatic` event"); the user prompt is unchanged (`payload()` is `{}`). The two committed system-prompt **goldens** (`system-no-tool.txt`, `system-tool.txt`) are regenerated (each gains exactly the new `migration` schema line + constraint bullet); the **`user.txt` golden stays byte-identical** (no new user-payload keys — verified by checksum).
+  - **Tests.** New `server/abilities/migration.test.ts` (**38 tests**): the kind/scale predicate sets; the three effect tables (exact keys / signs / monotonicity); `validateMigrationOps` (shape / leniency / drops / field caps); `shiftRelationTowardPositive` (one-step nudge, caps, war-untouched, missing→neutral, unknown no-op); `applyMigrationOp` (small / moderate / large source & destination dips, the `large` stability strain, name-based resolve, the relation nudge by kind, war-untouched, exodus-only-source, self-flow drop, unknown / ambiguous source & destination drop, 0-region direct-dip persistence, territory-untouched, clamping at 0 and 100); and the ability const contract (field, no `stateFields`, fresh `default()`/`stub()`, `isMeaningful`, `payload() {}`, `validate` wrapping, `apply` ctx wiring). `registry.test.ts` is updated for the **10-ability** contract (order, distinct fields, `fullDefaultTurn()`, `meaningfulFor`, re-export pin); the goldens are regenerated (verified idempotent).
+  - **Design note (interpretation of the issue body).** The Gitea #64 body sketched a simpler `flows: { type: 'refugees' | 'settlers' }` shape that included a settler **re-claim of no-man's-land**; the refined, approved design implemented here is the richer `migration.ops` union of **four nation-to-nation population-flow kinds** (migration / refugees / diaspora / exodus) — a purely **stat** layer that **never moves territory** (sole ownership stays untouched; a no-man's-land re-claim is deliberately out of scope, as territory movement remains the `regionChanges` lever's job).
+- **Guarantees.** No client (`src/`) changes, no new runtime deps, `shared/` keeps zero runtime deps, and no `@turf`/`@types/geojson` in any `.ts`. All unit tests green (additive); e2e (CI, `LLM_STUB=1`) unaffected (the stub is migration-inert).
+- **Version** bumped `0.41.0 → 0.42.0`.
+
 ## v0.41.0 — 2026-10-16 09:07
 - **#63: Disasters & pandemics — a `TurnResult.disasters` lever letting the LLM declare natural/man-made disasters (earthquake / flood / volcanic / wildfire / famine / plague / epidemic) on a nation or a region, with a deterministic stat dip (stability / economy / military).** It is a **turn-transient** feature, exactly like the v0.34 `hybrid` layer: **no new `GameState` field, no `db.ts` change, no durable list** — the ops are consumed inside `applyTurnResult` and leave nothing behind. It lands as the **9th ability** of the v0.40.0 modular system, in a new `server/abilities/disasters.ts` module, **registered last** in the `ABILITIES` registry (after `ideology`, before the loop appends `markers` / `nationOps`).
   - **Types (`shared/protocol.ts`).** New `DisasterType` (`'earthquake' | 'flood' | 'volcanic' | 'wildfire' | 'famine' | 'plague' | 'epidemic'`), `DisasterSeverity` (`'minor' | 'moderate' | 'severe'`), `DisasterEvent` (`{ type; target; severity; detail? }` — `target` is a nation ref **or** a region key), and `DisastersOps` (`{ events: DisasterEvent[] }`); `TurnResult.disasters` is **required**, placed between `ideology` and `markers`, default `{ events: [] }`.