The per-runtime-instance table of type descriptors. Descriptors
drive the derived operations and type-directed JSON decode;
registration is used by the interpreter now and compiled code later
(plan 01).
A dense, per-runtime-instance index into the type registry (resolved
decision, impl plan §1). Ids are not stable across runs; anything
persistent identifies types by name (filename is identity, design
§4.3).
The type of a field, payload, or element, as the JSON layer and the
derived operations need to see it. Named covers records, sums, and
opaque types — including recursive references.