Three fixes to match retail CLandBlock::get_land_scenes (0x00530460):
1. Loop bound: iterate 9×9 vertices (side_vertex_count=9), not 8×8
cells. Edge vertices (x=8 or y=8) produce valid spawns when the
per-object displacement shifts the position back into [0, 192).
Confirmed by named retail decomp do-while condition, WorldBuilder
vertLength=9, ACViewer Terrain.Count=81, AC2D wTopo[9][9].
2. Building suppression: check at the DISPLACED position's cell
(CSortCell::has_building per spawn), not at the loop vertex index.
Matches WorldBuilder buildingsGrid[gx2, gy2] pattern.
3. Slope filter: replace finite-difference gradient approximation
with triangle-aware normal sampling via new static method
TerrainSurface.SampleNormalZFromHeightmap. Picks the correct
triangle via IsSplitSWtoNE, matching retail find_terrain_poly →
polygon->plane.N.z and WorldBuilder's GetNormal().
Tests: 5 new tests for SampleNormalZFromHeightmap (flat=1.0, sloped<1,
cross-validates with SampleSurface instance method) and DisplaceObject
edge-vertex validity.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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