Add build identity reads for PE, ELF and Mach-O

[ero-qt]

First piece of #141: reading a module's build identity out of mapped memory, so offsets can later be keyed to exact builds instead of version heuristics.

• pe::DebugId: GUID and age from the CodeView debug directory entry, the pair symbol servers key on.
• elf::build_id: the NT_GNU_BUILD_ID note.
• macho::uuid: the LC_UUID load command.

Also introduces the first unit tests in the crate: a small mock host implementing the env process imports over in-memory buffers, so the readers run against hand-laid images on the host.

Since there's no precedent for tests here, happy to restructure if you'd prefer a different shape, like tests split into their own files or another home for the mock.