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.

config/onion-perf/mid/options.txt

@@ -0,0 +1,11 @@
+version:3955
+renderDistance:8
+simulationDistance:8
+biomeBlendRadius:1
+mipmapLevels:2
+entityDistanceScaling:0.75
+particles:0
+entityShadows:true
+graphicsMode:1
+maxFps:120
+enableVsync:true