Rework RecGroupBuilder API per review feedback

Replace the public surface with a fluent nested-builder API:

- Use a single kind-agnostic `PendingType` handle instead of separate
  `PendingStructId`/`PendingArrayId`/`PendingFuncId`.
- Drop the public `*Template` value types. Concrete fields/params take the
  existing `FieldType`/`ValType` directly; forward references to in-group
  siblings use `forward_ref_field`/`forward_ref_element`/`forward_ref_param`/
  `forward_ref_result`, which return small builders configured with
  `mutability`/`nullable` and committed with `finish` (a builder so that
  future ref attributes such as `shared` can be added without breaking the
  signature).
- Finality and supertypes are set via builder methods: `finality(Finality)`,
  `supertype(<concrete>)` for already-registered supertypes, and
  `forward_supertype(PendingType)` for in-group siblings. `build()` performs
  the same structural/finality validation the one-off constructors do, after
  registration so that forward supertypes resolve.

Getters are `get_struct`/`get_array`/`get_func -> Option<..>`.

Author: somdoron