chore(repo): archive historical docs under Docs/archive, drop tracked pycache

Move closed audits, task briefs, design rounds, perf reports, the OPSTACK
plan/progress logs and WORK-NEXT.md into Docs/archive/ (git mv, history kept).
Whitelist Docs/ in the strict .gitignore, ignore Tools/**/__pycache__/, and
remove the tracked .pyc. Repoint references in the kept docs and in non-test
source comments to the new paths.

.gitignore

@@ -12,8 +12,13 @@
 # The guarded single-entry test harness is source, not generated output.
 !/Tools/
 !/Tools/**
+/Tools/**/__pycache__/
+
+# Closed / historical documents.
+!/Docs/
+!/Docs/**
 
 !VoxelForge.uplugin
 
-# Keep every design / doc markdown at any depth (AUDIT P1 — these were untracked)
+# Keep every design / doc markdown at any depth
 !*.md