docs: client-perf analysis, bench harness and work journal

Everything the next agent needs to run the Phase 2 optimisation loop on a
machine that can launch the client:

- analysis/client-perf/: LEVERS (ranked levers + corrections), BENCH-MATRIX
  (world, scenes, config IDs, blocks B0-B14 with decision rules),
  PLAYER-TUNING, SPAWN-TUNING, research briefs, the jar scanner and its
  output, observed default configs, and proposed-configs/ (build.py
  generates tiers and bench variants and fails on unknown keys;
  apply_variant.py applies a variant and merges options.txt).
- analysis/client-perf/bench/onion-bench/: bench-world datapack (scene
  teleports, Create grid, mob pen, sound storm, 120 s teleport tour).
- analysis/audit/: the static recipe/dependency audit, including gen_fixes.py,
  which generates the shipped Paxi datapack and was never tracked.
- phase0/: per-machine measurement harness (Steam Deck, 7700K/1060,
  6700K/1080 Ti).
- .pi/journal.md: running work log; read it first.

All of it is packwizignored, so nothing here ships to players.

analysis/client-perf/proposed-configs/apply_variant.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""Apply a generated variant to a Minecraft instance (game closed!).
+
+    python3 apply_variant.py <ID> <path/to/.minecraft>     # e.g. C1, dh-r64, T-deck
+
+Copies every config file of out/<ID>/ over the instance. options.txt is
+*merged* key by key, so GUI scale, resolution, keybinds etc. survive variant
+switches. Prints what changed. Stdlib only; works on Windows too.
+"""
+import shutil
+import sys
+from pathlib import Path
+
+HERE = Path(__file__).resolve().parent
+
+
+def merge_options(src, dst):
+    new = dict(l.split(":", 1) for l in src.read_text().splitlines() if ":" in l)
+    lines = dst.read_text().splitlines() if dst.exists() else []
+    seen, out, changed = set(), [], []
+    for l in lines:
+        k = l.split(":", 1)[0]
+        if k in new:
+            seen.add(k)
+            if l != f"{k}:{new[k]}":
+                changed.append(f"{k}:{new[k]}")
+            l = f"{k}:{new[k]}"
+        out.append(l)
+    for k, v in new.items():
+        if k not in seen:
+            out.append(f"{k}:{v}")
+            changed.append(f"{k}:{v} (added)")
+    dst.write_text("\n".join(out) + "\n")
+    return changed
+
+
+def main():
+    if len(sys.argv) != 3:
+        sys.exit(__doc__)
+    vid, game = sys.argv[1], Path(sys.argv[2])
+    src = HERE / "out" / vid
+    if not src.is_dir():
+        sys.exit(f"{src} missing - run build.py first")
+    if not (game / "mods").is_dir():
+        sys.exit(f"{game} doesn't look like a .minecraft dir (no mods/)")
+    for f in sorted(p for p in src.rglob("*") if p.is_file()):
+        rel = f.relative_to(src)
+        dst = game / rel
+        if rel.as_posix() == "options.txt":
+            for c in merge_options(f, dst):
+                print(f"options.txt  {c}")
+            continue
+        dst.parent.mkdir(parents=True, exist_ok=True)
+        shutil.copyfile(f, dst)
+        print(f"copied       {rel.as_posix()}")
+    (game / "BENCH_VARIANT.txt").write_text(vid + "\n")
+    print(f"applied {vid}; recorded in BENCH_VARIANT.txt")
+
+
+main()