Data::Metadata: assert on field allocation; array size assertions.

Error,
IO/Defs,
Data::Metadata,
Object/Defs:
	switched to enum_cast() for enum-dependent array size assertions.
Cmd::Stage: added make_stage_shadow(); added copy IDFields::assign().

doc: tidy.
Hive/Defs, Node/Defs, Rule/Defs: alias Object::NULL_ID as with ID.
Object::Unit: added children property¹, parenting helpers, and is_identified().

1: This a (runtime) collection to enable object & slug traversal
and parenting strictness.
System::Driver: removed hive_order property; added find_hive().

System::Driver:
	added property accessors;
	removed from 'driver' doc-group;
	doc tidy.
System::Context: removed Type::untracked, added get_hive_pair().

System::Context: added is_host() and is_client().