TDD: PakKeyTests + PakFormatTests written first (confirmed a compile
failure against the not-yet-existing AcDream.Content.Pak namespace),
then PakKey (64-bit type:u8|fileId:u32|reserved:u24 compose/decompose)
and PakFormat (64-byte PakHeader, 24-byte PakTocEntry) implemented to
the normative layout in the MP1b plan. 21 tests green, including a key-
ordering test proving ascending numeric key order equals ascending
(type, fileId) tuple order (the TOC binary-search precondition) and an
explicit byte-offset test for both structs.
