Preserve Doorstop injection for Valheim macOS Rosetta launches

[No898]

Summary

Fixes #21.

This is a small macOS workaround for current Valheim/BepInExPack users on Apple Silicon.

When the Valheim start script runs the x86_64 game slice through Rosetta, Doorstop can fail to inject because the DYLD_* variables used for libdoorstop_x64.dylib are not present in the final Valheim process. The result is confusing for users: Valheim launches normally, but BepInEx never creates BepInEx/LogOutput.log and no plugins load.

This patch keeps the change limited to scripts/Start scripts/start_game_bepinex.sh:

• adds a full doorstop_path
• for macOS x64 launches, enters an x86_64 shell first
• sets DYLD_LIBRARY_PATH and DYLD_INSERT_LIBRARIES inside that shell
• then execs the Valheim executable

This is intentionally narrower than #20. It does not attempt native arm64 support or broader runtime changes; it is just a compatibility workaround for the current packaged Valheim launch path.

Local validation

Validated on:

• macOS 26.3.1 (25D2128)
• Apple M4
• Valheim 0.221.12 (network version 36)
• BepInExPack_Valheim 5.4.2333

Before the workaround:

• Valheim launched.
• BepInEx/LogOutput.log was not created.
• No plugins loaded.

After the workaround:

[Message:   BepInEx] Chainloader started
[Info   :   BepInEx] 11 plugins to load
[Message:   BepInEx] Chainloader startup complete

Notes

I opened this as a small PR because the broader Apple Silicon work in #20 is still draft. If you would rather fold this into that PR, I am fine with closing this.

This patch was prepared with AI assistance from Codex and validated locally on the machine above.

[arrowmaster]

Changes to the shell script should be submitted upstream to https://github.com/neighTools/UnityDoorstop/ where they can propagate to upstream BepInEx and then eventually all game specific repacks.