Deletes:
- TerrainChunkRenderer.cs (454 lines, replaced by TerrainModernRenderer)
- TerrainRenderer.cs (247 lines, older sibling, no production users)
- terrain.vert / terrain.frag (replaced by terrain_modern.{vert,frag})
Removes the temporary Task 8 perf-benchmark toggle (ACDREAM_LEGACY_TERRAIN
env var, _useLegacyTerrain field, parallel _terrainLegacy renderer
instance, [TERRAIN-DIAG/modern|legacy] label suffix). The modern path
is now the only path. Mirror N.5's mandatory-modern amendment: missing
GL_ARB_bindless_texture throws NotSupportedException at startup
(already in place via the BindlessSupport.TryCreate gate).
Three load-bearing research comments preserved verbatim from terrain.vert
into terrain_modern.vert before deletion: the MIN_FACTOR = 0.0 N-dot-L
floor block (cross-ref Lambert brightness split), the aPacked3 bit
layout, the gl_VertexID corner-table 2026-04-21 ConstructPolygons fix.
Also retires the now-orphaned _shader field (legacy terrain pipeline
was its only user).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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