docs(research): round two -- wider search, AVX2 integer probe, crater transaction design

Fork of the Sol research session with the owner's answers (player-only 1 s collision; bounded
digging but fast crater spells). AVX2 integer kernel 4.25-5.86x over its scalar reference with
exact parity; radius-10 m crater = 714 bricks, cached-field edit < 1 ms (render not measured).
Cost estimate revised to 6-12 engineer-months; kill test gate <= 0.35 s worker wall on real plans.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Saved/Research/round2_probe-results.txt

@@ -0,0 +1,31 @@
+Machine: Ryzen 7 5800X3D, Windows, MSVC 2022, 2026-09-29.
+Command from Saved/Research:
+cmd /c '"E:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\Build\vcvars64.bat" >nul && cl /nologo /O2 /std:c++20 /EHsc /arch:AVX2 round2_probe.cpp /Fe:round2_probe.exe'
+.\round2_probe.exe
+
+First run:
+full trial=0 scalar_ms=492.007 avx2_ms=101.294 speedup=4.857 mismatch=0
+full trial=1 scalar_ms=496.238 avx2_ms=106.015 speedup=4.681 mismatch=0
+full trial=2 scalar_ms=469.590 avx2_ms=92.511 speedup=5.076 mismatch=0
+crater radius_vox=40 touched_bricks=714 tile_AABB_max=64 samples_AABB=531441
+crater copy_ms=1.623 edit_ms=0.865 changed_lattice=94843 copy_bytes=33554432
+
+Second run after dead-code cleanup (same evaluation code):
+full trial=0 scalar_ms=430.286 avx2_ms=89.209 speedup=4.823 mismatch=0
+full trial=1 scalar_ms=456.202 avx2_ms=107.464 speedup=4.245 mismatch=0
+full trial=2 scalar_ms=447.117 avx2_ms=94.584 speedup=4.727 mismatch=0
+crater radius_vox=40 touched_bricks=714 tile_AABB_max=64 samples_AABB=531441
+crater copy_ms=1.422 edit_ms=0.914 changed_lattice=94843 copy_bytes=33554432
+
+Third run after adding a scattered direct-query measurement (same field evaluation code):
+full trial=0 scalar_ms=478.627 avx2_ms=85.763 speedup=5.581 mismatch=0
+full trial=1 scalar_ms=471.015 avx2_ms=80.318 speedup=5.864 mismatch=0
+full trial=2 scalar_ms=444.320 avx2_ms=82.728 speedup=5.371 mismatch=0
+random_query trial=0 count=200000 ms=63.030 ns_per_query=315.1 checksum=-1501231
+random_query trial=1 count=200000 ms=62.785 ns_per_query=313.9 checksum=-1501231
+random_query trial=2 count=200000 ms=62.799 ns_per_query=314.0 checksum=-1501231
+crater radius_vox=40 touched_bricks=714 tile_AABB_max=64 samples_AABB=531441
+crater copy_ms=1.394 edit_ms=0.861 changed_lattice=94843 copy_bytes=33554432
+
+Both MSVC builds succeeded. The vcvars script emitted a missing-vswhere.exe notice.
+Synthetic workload; no Unreal build/launch or GPU benchmark.