This commit updates the `v2::Receiver` to be generic over its
typestate (e.g., `UncheckedProposal`, `InputsSeen`, etc.). The
motivation:

* Improves readability and maintainability: Each typestate now
  clearly belongs to the receiver session. This makes the code easier
  to reason about, without runtime cost. It also sets us up to
  define a shared pattern between v2, NS1R and future state machines.

* Decouples typestate data from transition logic: NS1R typesates can now
  consume state structs without being tied to the transition machinery.
  This keeps state transitions encapsulated and makes the underlying
  session data easier to use in other contexts.

This commit renames the original `Receiver` struct to `WithContext`.
This type now represents the receiver session once the HPKE context has been
created. The Receiver name is re-purposed for the new generic wrapper
over typestate.

For the UniFFI exported receiver we monomorphizes the exported structs over a
specific typestate. UniFFI doesn't support exporting generic structs, so
we expose a concrete type to the FFI.