feat: sub-region-level territory — ownership, LLM and map at region granularity (v0.19.0)

Move territory from whole-country (ISO_A3) to individual sub-region id
granularity (PR 3 of #16):

- new server/regions.ts (turf-free): loadRegions (boot), expandTerritory
  (game creation), migrateTerritoryToRegions (legacy games on db.load),
  regionNamesFor / regionLabel (LLM prompt), graceful ISO_A3 fallback
- game creation expands each preset nation's ISO_A3 list to its sub-region
  ids; pre-v0.19 saved games migrate transparently on read (idempotent)
- LLM contract: territoryChanges gain/lose sub-region ids (one region at a
  time; gain moves out of the prior owner; lost-without-gainer = unowned);
  user prompt carries a top-level regions reference table
  ({id: 'name (parent)'} over the state's territory ids)
- WorldMap: choropleth fill per sub-region (countries can split between
  nations), ownership via properties.regionId through the shared
  mapFeatureOwner, flash lights exactly the changed features
  (featuresByRegionIds)
- deterministic stub: one region-level move per non-empty round (player
  annexes the lexicographically-first region of the first other nation)
- tests: new server/regions.test.ts (real geo + fallback + seams);
  map/engine tests re-pinned to region ids; scripts/regions.test.ts stays green

Measured full wwii-1939 user prompt (411 regions): ~24.7k chars / ~6.2k
tokens — far under the 80k frontier limit, so the full regions table is
always sent (no frontier-limited mode needed).

Part of #16

CHANGELOG.md

@@ -2,6 +2,16 @@
 
 All notable changes to **History** are listed here, newest first.
 
+## v0.19.0 — 2026-10-13 22:02
+- **Sub-region-level territory (PR 3 of #16): ownership, the LLM, and the map all operate at individual sub-region granularity instead of whole-country.**
+  - `Nation.territory` is now a list of **sub-region ids** (`FR-57`, `DE-BB`, `US-AK`, … — the feature ids of `regions.geo.json`) rather than ISO_A3 country codes. At game creation (`POST /api/games`) each preset nation's ISO_A3 list is expanded to all its sub-region ids by the new `server/regions.ts` (a turf-free module that loads `public/`/`dist/regions.geo.json` once at boot into a region-id index + a country→regions map). Games saved before v0.19 are transparently migrated to region-level territory on read (`db.load` → `migrateTerritoryToRegions` — idempotent, and a no-op when the geo is not loaded).
+  - **The LLM contract follows the map:** a `TurnResult`'s `territoryChanges` now gain/lose sub-region ids, so a border can shift **one region at a time** — a gained region is moved out of whichever nation currently holds it, and a lost region with no matching gainer becomes unowned no-man's-land (the `applyTurnResult` splice logic was already format-agnostic and is unchanged). The user prompt carries a new top-level `regions` reference table — `{ id: "name (parent)" }` for every region id present in the world state — so the model can map an id it is considering to a real place; the system prompt's hard constraints now name sub-region ids (e.g. `FR-57`). **Prompt-size measurement (full `wwii-1939`: 7 nations, 411 regions): the complete user prompt is ≈ 24.7k chars / ≈ 6.2k tokens at start (≈ 26.4k / ≈ 6.6k with 8 recent events + a 2-item round) — far under the ~80k frontier limit, so the full `regions` table is always sent (no frontier-limited mode was needed).**
+  - **The choropleth is now truly per-region:** a nation's color fills exactly the sub-regions it owns, so a single country can be visibly split between nations, and unowned sub-regions render grey. Click/hover ownership resolves via the feature's `properties.regionId` (now the RegionMap key) through the shared `mapFeatureOwner` in `shared/protocol`, and the territory-change flash lights up exactly the regions whose owner changed (`featuresByRegionIds`, replacing the country-expanding `featuresByParents`).
+  - **Graceful degradation:** when the geo file is missing or unreadable the region maps stay empty and `hasRegionData()` is false, so expansion, migration, the prompt's `regions` table, and ownership resolution all fall back to the pre-v0.19 whole-country ISO_A3 behavior — the server still boots and the app still works (with a one-time console warning).
+  - **The deterministic stub stays deterministic** and now also makes exactly one region-level move per non-empty round: the player annexes the lexicographically-first sub-region of the first other nation (by key order) — empty rounds make no move — so unit tests and e2e assert the exact region that changes hands.
+  - **Tests:** new `server/regions.test.ts` (real-geo expansion/migration, the not-loaded fallback path, and the fixture seams); `server/map.test.ts` and `server/engine.test.ts` re-pinned to region-id fixtures, including the #6 regression guard (owner from `properties.regionId`, never the repacked numeric `feature.id`) and new `applyTurnResult` tests for prior-owner transfer, no-owner gain, and no-gainer lose.
+- Part of #16
+
 ## v0.18.0 — 2026-10-13 18:54
 - **Fixed a stale-owner display bug: the region hover tooltip and the dossier "Selected" label no longer print the static parent-country code (e.g. "FRA").** The GeoJSON `properties.parent` is a *static geographic-origin* field that never updates when a country is taken, yet it was rendered in the hover tooltip (as a sub-line and as a title fallback) and in the dossier's `Selected:` label (as a `(FRA)` suffix) — so a sub-region of a transferred country still displayed the old country's code next to the (correct) new owner. The tooltip now shows only the region name + the live owner chip (from the RegionMap) + key stats, and the dossier label shows only the region name. Ownership resolution still keys on `properties.parent` (unchanged) — only the display of it was removed.