Redesign sexpt with GADT-based codec (Jsont soup paper)

Replace opaque closure record with a GADT that preserves codec
structure, following the approach from Buenzli's "An Alphabet for
Your Data Soups" paper:

- GADT constructors for each S-expression sort (Atom, List, Obj,
Any, Map, Rec, Variant, Pair, Triple, etc.)
- dec_fun GADT with Type.Id for unordered record member decoding
- Heterogeneous Dict for buffering typed member values
- Structural encode/decode by pattern matching on GADT
- New query/update API: get_mem, get_nth, update_mem, delete_mem

Internal redesign only — 'a t stays abstract, all existing tests
pass unchanged.