aead-value 0.6: rename FfiValue to Value, make it Clone and non_exhaustive, move transport's framing tags #371

Background

vitaminc-aead-value is the value model behind vitaminc's encryption traits:
FfiValue is a value whose type is known only at runtime, as it arrives from
JavaScript, Go or Python. Each scalar seals as a one-byte type tag plus its
payload, inside the encrypted envelope. That tag table is frozen, because
stored ciphertexts depend on it. The transport module is a separate encoding
for handing a value tree across WebAssembly memory; it is never stored.

Problem

1. The name misleads. FfiValue reads as "a type for FFI", but it is the
   value model for any vitaminc cipher. Reviewers have proposed moving the
   crate into Stack Encrypt on that basis.
2. It can't be cloned. Stack Encrypt's plan engine clones a field's
   plaintext per operation, so it wraps FfiValue in its own dynamic::Value
   only to add Clone (packages/stack-encrypt/src/dynamic/value.rs).
3. Adding a kind is a breaking change. FfiValue and ValueKind are
   exhaustive enums.
4. Transport's framing tags block the tag table. ARRAY, OBJECT and
   PASSTHROUGH are 0x10, 0x11 and 0x12 (transport.rs:27-35), and
   transport reuses the scalar tag numbers for its leaves. So the next scalar
   tags can't use 0x10 to 0x12 without colliding.

Proposal

1. Rename FfiValue to Value, keeping
   #[deprecated] pub type FfiValue = Value; for one release.
2. Implement Clone as a deep copy that rebuilds every leaf into a fresh
   Protected, as Stack Encrypt's duplicate() does today. Document that a
   clone is under the same custody as its original.
3. Mark Value and ValueKind #[non_exhaustive].
4. Move ARRAY, OBJECT and PASSTHROUGH to 0xF0, 0xF1 and 0xF2, in
   Rust and in Go's vcffi together. Transport carries no compatibility
   commitment, and every user of it ships with its peer.

Breaking; part of 0.6.0. Decided in ADR-0002 (cipherstash/stack#1139).

Relationship to other work

• aead-value: a cross-language conformance corpus for the FFI transport codec #332: the transport conformance corpus moves with step 4.
• aead-value: serde Deserializer for the transport value model #348 (serde for FfiValue) and aead / aead-value: after a decrypt, one unwiped copy of each decrypted string stays in freed heap memory #370 (plaintext left in the guest heap) touch
  the same type; a new copy from step 2 must stay inside Protected.