Large additional-generation fix

Symptoms

A moderately larger patch selection could appear to stop during generation.
A larger selection could return no visible preview or leave part of the selected area untouched.

app.js requested qualityTerrainAttempts: 1, but generatePatchAttempt() did not forward that option into the expansion candidate selector. An interactive click could therefore run extra terrain searches and a second complete production generation. On larger selections this could keep the patch worker busy long enough to look interrupted or be terminated under memory/CPU pressure.
Expansion candidates are production-sized (258 x 183) but patch selections had no corresponding upper bound. A selection larger than the candidate window caused source indices outside that fixed candidate to be skipped, so portions of a large selection could remain effectively ungenerated.
The first large-selection tiling prototype over-overlapped slightly oversized ranges, multiplying work. Tiling now evenly distributes the range with ~28-cell overlap.
Internal tile seams initially treated roads/rails created by an earlier tile as pre-existing portal contracts. A later tile could then hard-fail on a road that did not exist before the user's operation. Only transport present before the entire large patch is now protected by the hard portal contract.

Forward qualityTerrainAttempts to the expansion candidate selector. Interactive generation now actually performs one requested full candidate per click.
Keep selections up to 258 x 183 on the existing single-candidate fast path.
Automatically split larger Expansion selections into overlapping production-sized tiles, including freeform/lasso selections via polygon clipping.
Process tiles outward from already-generated geography and merge all tile results into one preview world.
Soft per-tile quality floors may use best-available results; hard seam failures still reject the operation.
Preserve only the pre-operation road/rail set as a hard seam portal baseline across internal tiles.
Worker-owned previews avoid an unnecessary second large field snapshot; synchronous/API calls retain atomic rollback snapshots.

Direct near-limit patch 250 x 180: PASS, one full candidate, not tiled.
Large patch 280 x 200: PASS, 4 tiles, 56,000/56,000 selected cells covered by generated records, seam clean.
Worker large patch 280 x 200: PASS, outer worker result and inner patch result both successful, 4 tiles, seam clean.
STEP17 validation: PASS for Alternative variants 0 and 1; variants remain distinct.
STEP15 validation: PASS; Expansion seam clean, escaped footprint cells 0, unresolved road/rail portals 0.
Previous reported-geography regression: PASS; river-on-sea 0, duplicate capitals 0, tiny generated prefectures 0, admin/prefecture frontier breaks 0, established-frontier max elevation jump 0.0268.