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.

onion-perf-setup.bat

@@ -0,0 +1,7 @@
+@echo off
+rem Onion MC: apply a performance tier (deck / mid / high) to this instance.
+rem Double-click it in the instance folder, or run: onion-perf-setup.bat high --yes
+powershell -NoProfile -ExecutionPolicy Bypass -File "%~dp0onion-perf-setup.ps1" %*
+set ONION_RC=%ERRORLEVEL%
+pause
+exit /b %ONION_RC%