fix(cream.sh): work from any CWD, MangoHud support, bitness-aware preload, CREAM_CONFIG_PATH as file or dir

[gongarn]

Summary

Four small, self-contained fixes for package/cream.sh that address open issues:

1. Work from any working directory (Crusader Kings III launch options not working for Steam #62) — Steam may launch the game from a different CWD, breaking sh ./cream.sh %command% launch options while terminal runs worked. The script now cds to its own directory first.
2. MangoHud support (does not work with mangohud #51) — sh ./cream.sh mangohud %command% sets MANGOHUD=1 and MANGOHUD_DLSYM=1 so both preloads coexist (previously only one took effect).
3. Bitness-aware preload (Issue with LD_PRELOAD on CK3 #71) — reads the ELF class of libsteam_api.so (byte at offset 4: 01 = 32-bit, 02 = 64-bit) and preloads only the matching creamlinux library, removing the wrong ELF class noise from ld.so visible in CK3 logs. Unknown files fall back to preloading both.
4. CREAM_CONFIG_PATH — now accepts both a file path and a directory containing cream_api.ini (the script checked it as a directory while the C++ loader reads it as a file path).

Testing

• All four scenarios verified on a real Steam setup (launch from foreign CWD, mangohud prefix, 32/64-bit fake libsteam_api.so, config as file/dir).
• bash -n clean; behavior of the non-modified parts unchanged.