TypeDef:Uint64 was never specified in any ADR and was a partial, incomplete
addition: type_size() returned None (so is_fixed_size() was false),
OffsetMap::compute rejected it, LayoutBuilder::build would unreachable!()
panic on it, and the validator did not register a TypeDef:Uint64 keyword.
Only the SequentialReader path worked, and only because it dispatched
directly to data_access::read_u64 without consulting type_size().
Remove the variant from TypeDefKind, the read_u64/write_u64 primitives,
FieldValue::U64, the sequential-reader and engine dispatch arms, and the
associated tests. The engine now has exactly 17 first-class kinds, matching
the spec. All 286 tests pass; the workspace builds clean.
glm-5.2
14d9cf281f