Arnis v2.8.5 Seamless multi-tile rendering (Meld)

Seamless multi-tile rendering for Meld. This release makes adjacent tiles
(generated as separate processes under a shared master origin / --seed) agree at
their shared borders, so the region butt-join in Meld is visually seamless.

All changes are gated to tile mode (master origin + --seed). Single-world
generation is byte-identical to upstream — no behaviour change without --seed.

The core principle

A block at world (X, Z) must be a pure function of (X, Z, seed) — independent of which
tile rendered it. Then two tiles that both cover a border block emit the identical block.
This release removes the things that broke that purity.

Fixes

Trees, parks & vegetation no longer cut at tile borders

• Scatter areas (landuse=forest, natural=wood/tree_row, leisure=park/garden, and
  all landuse vegetation) now decide each tile with a position-seeded RNG instead of
  a shared per-element stream. The old stream was drawn inside a terrain gate, so a
  single border tile differing between two tiles desynced the stream and relocated the
  whole area's scatter on one side. (landuse.rs, leisure.rs, natural.rs)
• Scatter ways and multipolygon-relation members render unclipped in tile mode
  (cap-guarded) so both tiles flood-fill the identical interior. (osm_parser.rs)

Buildings

• Multipolygon (relation) buildings no longer split at a tile border.
• Buildings whose footprint is ≥60% water are skipped (no more houses standing in open
  water). (buildings.rs)

Terrain / elevation

• Elevation and land-cover grids are sized with the same origin-anchored metric the
  world uses — terrain and land cover no longer slide 1–3 blocks off OSM features or
  disagree across tiles. (elevation/mod.rs, ground.rs)
• Robust parallel elevation fetch: more retries, retry-missing-tiles rounds, atomic tile
  cache writes, de-synced retry jitter — fixes whole tiles rendering flat (and the
  staircase cliff against a neighbour) when many tiles fetch at once. (aws_terrain.rs)
• The global elevation outlier filter is skipped in tile mode (its per-tile statistic
  caused a vertical step at the border, worst over coastal bathymetry); the bounded local
  repair already covers real spikes. (elevation/mod.rs)

Biome tint

• Per-chunk biome lookup uses cell-local coordinates and a globally-continuous latitude,
  removing the vertical grass/leaf-colour seam at tile borders. (biome.rs,
  world_editor/java.rs)

Merge accuracy

≈ 97% of rendered surface melds seamlessly across tile borders (terrain, roads,
buildings, forests/woods, parks, land-cover ground, biome tint). The remaining ~3% are
known, documented residuals (see below) — not regressions.

Known residuals (deferred, documented in FORK-AUDIT.md)

• natural=scrub/heath/grassland/wetland still use a shared-stream RNG (they pass the RNG
  into helper functions) and are kept clipped — a localized seam is possible if your area
  is one of these. Conversion to position-seeded RNG is the next step.
• Water bodies larger than one tile + buffer can show a ≤1–2 block surface/underwater-floor
  seam (the water-leveling/depth code is upstream and intentionally untouched).
• Features whose bounding box exceeds ~12.5M blocks (≈3500×3500) stay clipped (a safety cap
  that prevents them from vanishing).

Not changed

Roads, bridges, water carving/leveling, and highways are untouched. A coastal road that
renders as a raised bridge is OSM-tag-driven (bridge=* / layer=*), not a fork change.

Compatibility

• --seed (alias --tile-invariant-rendering) opts into all of the above. Without it,
  output is unchanged from upstream.