Re-coarsen RU-ARK Arctic-coast band into RU-PEO + RU-YAM (#41)

#41: re-partition the RU-ARK mainland's 28-part lat>=68 Arctic-coast band
into two new regions via a new turf-free, idempotent pipeline stage
scripts/split-arkhangel-band.mjs (npm run split:regions:band), mirroring the
#31 split:

- RU-PEO (Pechora, 13 parts, ~37,678 km2) -> key pechora-qwhs
- RU-YAM (Yamal, 15 parts, ~51,722 km2) -> key yamal-gosw
- RU-ARK keeps its id/name/key arkhangel-sk-lhbk (ownership-safe); the 5-part
  lat<68 core (~305,229 km2) stays RU-ARK.

Pure re-partition: no geometry moved (byte-identical union). The two new
features are inserted immediately after RU-ARK-SZ. Manifests regenerated with
the real scripts (1,798 -> 1,800 keys + adjacency; adjacency pairs 4,471 ->
4,473). Fixed-point dataset unchanged except feature count (11,794 parts /
11,795 rings / 164,801 verts / 1 KZ-KZY hole). Four dataset/manifest tests
updated (RUS 82 -> 84 sub-regions). Version 0.25.0 -> 0.26.0.

Fixes #41

scripts/region-adjacency.test.ts

@@ -34,7 +34,7 @@ describe('region-adjacency.json', () => {
     const adjacencyKeys = new Set(Object.keys(adjacency));
     expect(adjacencyKeys).toEqual(manifestKeys);
     expect(Object.keys(adjacency)).toHaveLength(keys.size);
-    expect(adjacencyKeys.size).toBe(1798);
+    expect(adjacencyKeys.size).toBe(1800);
   });
 
   it('every neighbour is itself a valid region key (referential integrity)', () => {
@@ -85,24 +85,30 @@ describe('region-adjacency.json', () => {
     expect(adjacency['baden-wurttemberg-oyje']?.includes('moselle-rygy')).toBe(true);
   });
 
-  it("the #31 island splits: New Earth is isolated; Severnaya Zemlya touches only the Arkhangel's main ring", () => {
+  it("the #31 + #41 Arctic splits: New Earth isolated; Severnaya Zemlya, Pechora and Yamal adjacency", () => {
     // v0.25 (#31) split RU-ARK into the mainland (arkhangel-sk-lhbk), the
     // Franz Josef Land islets (new-earth-eikt) and the Severnaya Zemlya
     // islets (severnaya-zemlya-awqp). New Earth sits ~100+ km from any land
-    // → a genuinely isolated island (empty list). Severnaya Zemlya's
-    // digitized islet rings share boundary vertices with the coarsened
-    // mainland ring (a pre-existing intra-feature contact, exposed by the
-    // split) → it touches exactly its former parent, with no overlap area.
+    // → a genuinely isolated island (empty list).
+    // v0.26 (#41) repartitioned the mainland: the 5 lat < 68 parts stay
+    // arkhangel-sk-lhbk, and the 28 lat ≥ 68 band parts became pechora-qwhs
+    // (the SW cell) and yamal-gosw (the NE cell). The Severnaya Zemlya
+    // islets' digitized rings touch the NE band part (now yamal-gosw), not
+    // the trimmed core, so SZ now borders Yamal (its former mainland contact
+    // moved to Yamal). The trimmed core keeps its real mainland neighbours
+    // but loses the SZ contact. Pechora (SW) borders the existing Nenets
+    // region to the south and Yamal to the north.
     expect(adjacency['new-earth-eikt']).toEqual([]);
-    expect(adjacency['severnaya-zemlya-awqp']).toEqual(['arkhangel-sk-lhbk']);
-    // the mainland keeps its six real neighbours plus the island contact
+    expect(adjacency['severnaya-zemlya-awqp']).toEqual(['yamal-gosw']);
+    expect(adjacency['pechora-qwhs']).toEqual(['nenets-qyga', 'yamal-gosw']);
+    expect(adjacency['yamal-gosw']).toEqual(['pechora-qwhs', 'severnaya-zemlya-awqp']);
+    // the trimmed core keeps its real neighbours (the band's SZ contact moved to Yamal)
     expect(adjacency['arkhangel-sk-lhbk']).toEqual([
       'karelia-ohem',
       'kirov-gfvm',
       'komi-eoeh',
       'murmansk-etas',
       'nenets-qyga',
-      'severnaya-zemlya-awqp',
       'vologda-ynvr',
     ]);
   });