feat(perf): add one-step tier setup script for existing players

Default Options only seeds configs when absent, so existing players never
got the tuned settings. build.py --ship now also writes the Deck/Mid/High
tier files to config/onion-perf/<tier>/, and onion-perf-setup.{sh,ps1,bat}
(installed into the instance folder) backs up and replaces the tier's
config files and merges only the tuned video keys into options.txt,
keeping keybinds, audio and vsync/max-FPS choices.

PLAYER-TUNING: point players at the script; bench B4 GC verdict (G1 for
everyone; ZGC/Shenandoah tested worse); 16 GB PCs 5-6 GB heap.

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

@@ -11,7 +11,8 @@ pinned jar's source (see README.md, "Key provenance").
 Output: out/<variant>/<path relative to the game dir>, plus out/MANIFEST.md.
 
     python3 build.py            # from this directory; stdlib only
-    python3 build.py --ship     # also copy T-default into the repo (see SHIP), then
+    python3 build.py --ship     # also copy T-default into the repo (see SHIP) and the
+                                # player tiers into config/onion-perf/ (see PERF), then
                                 # run `packwiz refresh` from the repo root
 """
 import json
@@ -48,6 +49,15 @@ SHIP = {
     "config/modernfix-mixins.properties": "config/modernfix-mixins.properties",
 }
 
+# Player tiers `--ship` also writes to config/onion-perf/<tier>/ (paths relative to
+# the game dir) for onion-perf-setup.sh/.ps1, which existing players run to apply a
+# tier over their configs. A tier carries only the files its layers tune, so
+# untuned files (sable-client.toml; sodium-options.json except on deck) keep the
+# player's own settings. modernfix-mixins.properties is enforced by packwiz already.
+PERF = "config/onion-perf"
+PERF_TIERS = {"deck": "T-deck", "mid": "T-default", "high": "T-high"}  # mid == pack default
+PERF_SKIP = {"config/modernfix-mixins.properties"}
+
 # game-dir path -> (base file, format, may_append_new_keys)
 FILES = {
     "options.txt": (TEMPLATES / "options.txt", "options", False),
@@ -384,6 +394,15 @@ def main():
             (REPO / dst).parent.mkdir(parents=True, exist_ok=True)
             shutil.copyfile(OUT / "T-default" / src, REPO / dst)
         print(f"shipped {len(SHIP)} files to {REPO / DO}; now run `packwiz refresh`")
+        shutil.rmtree(REPO / PERF, ignore_errors=True)
+        for tier, vid in PERF_TIERS.items():
+            layers, _ = VARIANTS[vid]
+            names = sorted({n for layer in layers for n in layer} - PERF_SKIP)
+            for name in names:
+                dst = REPO / PERF / tier / name
+                dst.parent.mkdir(parents=True, exist_ok=True)
+                shutil.copyfile(OUT / vid / name, dst)
+            print(f"shipped tier {tier} ({vid}): {len(names)} files to {REPO / PERF / tier}")
 
 
 main()