The first basename port followed Zig 0.16's ComponentIterator-based
implementation (the system std); Ghostty 1.3.1 builds with 0.15.2. The
theme-name check now ports 0.15.2's basenamePosix/basenameWindows: a ':'
at byte 1 ends the drive designator for any first byte, so '1:theme'
yields 'theme'. Tests use the 0.15.2 cases.