mpq: Store File instead of BufReader

[nocthir]

Move Archive to store a File instead of a BufReader<File> and create BufReader instances on demand inside methods. This relaxes method receivers from &mut self to &self where possible (e.g., read_file, list), which enables sharing an Archive across threads and reading multiple files concurrently from the same archive. Updated benches, examples, and tests accordingly.

No breaking changes. Method receivers changed from &mut self to &self are source compatible; existing code using &mut self can still call these methods.

[nocthir]

Multiple BufReader reading from the same underlying file handle (including handles created via File::try_clone) is not safe logically: they share a single file cursor, so reads will interleave and corrupt each other’s view. The BufReader docs explicitly warn that “creating multiple instances of a BufReader on the same stream can cause data loss” because each keeps its own buffer while the shared cursor moves underneath.