# Final additional-generation stability fix

## Remaining failure reproduced
After the earlier large-selection work, a position-dependent rollback still existed. A large overlapping expansion could fail with `patch-large-final-seam-failed (road-portal-broken)` even though the road in question never crossed from established geography into newly generated geography.

The concrete false portal was an old `minorRoads` segment wholly inside the already-generated map. It crossed the synthetic write/blend band, so the diagnostic system treated it as a mandatory expansion seam portal. When that unrelated old road could not be rerouted through the patch gateway, the complete multi-tile operation was rolled back and the UI showed no additional-generation preview.

## Fix
Expansion transport portals are now contractual only when the pre-patch road/rail transition actually crosses **generated ↔ ungenerated** geography. A route whose two sides are both already generated (or both previously ungenerated) is not a user-visible expansion seam and is excluded from the hard portal contract.

The hard gate itself remains enabled. Genuine generated/ungenerated transport crossings are still audited; the change removes only false portals created by the implementation write band.

The previous large-selection stability changes are retained: bounded tiling, deferred internal-tile transport seam checks, final whole-selection repair/audit, single final administrative/terrain coherence passes, lightweight internal snapshots, and reduced duplicate per-tile work.

## Final verification
- 300×339 direct expansion: PASS, 4 tiles, seam clean.
- 500×350 direct expansion: PASS, 4 tiles, seam clean.
- 600×400 Worker expansion, deliberately bottom-right/off-center: PASS, 9 tiles, seam clean.
  - selected cells: 240,000
  - previously ungenerated selected cells: 192,786
  - missing after generation: **0**
- Exact position that reproduced the false road portal: PASS.
  - selected cells: 125,268
  - previously ungenerated selected cells: 106,629
  - missing after generation: **0**
  - broken road portals: **0**
- Previously reported geography regressions: PASS.
  - river points on sea: 0
  - duplicate prefectural capitals: 0
  - tiny newly-generated prefectures: 0
  - administrative frontier breaks: 0
  - max established-frontier elevation jump: 0.0268
- STEP15: PASS; expansion seam clean, no escaped footprint, unresolved road/rail portals 0.
- STEP17: PASS; alternatives still differ, explicit render revision and one-worker-per-patch behavior retained.
- Seam stress: 2 seeds × 4 directions = 8 cases, all seam clean; maximum observed frontier elevation jump 0.0364 vs hard limit 0.075.

## Island handling
Small disconnected components are not deleted solely because they are small. Sea-separated components without a valid adjacent land prefecture can represent legitimate islands and remain preserved.