Two bugs in loadPatchArchives():
1. isLetterPatch detection was inverted (rfind != 0 is false for all
"patch-*" entries), making the disable flags non-functional
2. Patch file lookup used exact std::filesystem::exists() which is
case-sensitive on Linux — Patch-A.MPQ wouldn't match patch-a.mpq
Now scans the data directory once and builds a case-insensitive lookup
map, so any case variant (Patch-A.MPQ, patch-a.mpq, PATCH-A.MPQ) is
found correctly.
Loads speech-enUS.MPQ, expansion-speech-enUS.MPQ, and lichking-speech-enUS.MPQ
which contain NPC voice/creature sound files. These MPQs were present but not
being loaded, causing all .ogg NPC greeting sounds to show as NOT FOUND.
