feat(bench): add matrix runner, gamescope and per-session controls

- bench.py: gamescope-headless display (default, 1920x1080; ~2% cost vs
  desktop for isolation from the shared Wayland desktop), --shader,
  --dh, --disable-mods/--add-mods, --world-zip, crash-safe restore
  journal, spectator mode in the plan (elevated cameras fell to the
  sea floor and S4 measured underwater darkness)
- matrix.toml/matrix.py: declarative, resumable B0-B14 runner keyed on
  mods hash (warns when B0 must be re-run)
- build.py: bench-only resource-pack variants (C1-fa, C1-fa-ext,
  emf-lod*-fa, C1-bl); every bench variant pins resourcePacks so packs
  don't leak between sessions; shipped tier output unchanged
- analyze.py: tagged labels use their own block's baseline

.gitignore

@@ -7,3 +7,4 @@ immersive_paintings_cache/
 # Large or regenerable local artifacts
 *.tar.gz
 analysis/client-perf/proposed-configs/out/
+__pycache__/