phase 3.3, 3.6: cip-68 ref-nft pair + sign_partial primitive

new aldabra-core::cip68 module:
- asset name prefixes 100 (0x000643b0 ref) / 222 (0x000de140 user) /
  333 (0x0014df10 ft). prefixed() guards 32-byte total cap so caller
  can't blow past the cardano protocol limit by accident.
- json_to_plutus_data: serde_json::Value → PlutusData (recursive).
  numbers must fit i64. strings → BoundedBytes (cip-68 convention is
  bytes-keyed datum maps, not text). null is rejected, floats rejected.
- build_cip68_datum_cbor wraps the metadata in the canonical
  Constr 0 [meta_map, version_int=2, Constr 0 []] shape.

new aldabra-core::mint::build_signed_cip68_nft_mint:
- mints two assets simultaneously under one policy (ref + user, qty 1
  each), three outputs (ref @ ref_addr w/ inline datum, user @ user_addr,
  change). same two-pass fee refinement as the rest of the path.
- mutable nfts: pass ref_addr == change_addr. wallet's payment key can
  later spend the ref UTXO and re-create with new datum.
- immutable: caller passes an always-fails script address (phase 4
  concern; today this fn trusts whatever's passed).

new aldabra-core::sign module + add_witness:
- decodes a conway tx (any state — unsigned or partially signed),
  signs the body hash with the wallet's payment key, appends a
  VKeyWitness to the witness_set, re-encodes. body is invariant
  (regression test asserts the body hash before and after the witness
  append are identical).
- this is the missing primitive for n-of-k multisig flows: each party
  calls add_witness on the previous party's output cbor; any party
  submits via wallet.submit_signed_tx.

mcp tools: 10 → 12.
- wallet.mint.cip68_nft — args: user_address, name_body_hex (≤28b),
  metadata (json object), user_lovelace? ref_address? ref_lovelace?
  invalid_after_slot? — defaults provided for the ergonomic case
  (ref_addr=wallet, lovelace=1.5 ADA each).
- wallet.sign_partial — args: cbor_hex — appends our witness, returns
  updated hex. usable for MAP treasury 2-of-2 once a
  wallet.mint.unsigned-with-policy-arg lands (TODO, deferred).

65 → 79 unit tests. cip68 module: 9 tests covering prefix+datum
shape. sign module: 4 tests covering one-witness, two-witness,
body-hash invariant, garbage rejection. integration test in mint
verifies cip68 build produces 3 outputs with inline datum on the
ref output.

crates/aldabra-core/src/lib.rs

@@ -35,13 +35,22 @@ use pallas_addresses::{
 use thiserror::Error;
 use zeroize::ZeroizeOnDrop;
 
+pub mod cip68;
 pub mod derive;
 pub mod metadata;
 pub mod mint;
+pub mod sign;
 pub mod tx;
+pub use cip68::{
+    build_cip68_datum_cbor, ft_asset_name, ref_nft_asset_name, user_nft_asset_name,
+};
 pub use derive::{derive_payment_key, derive_stake_key, PaymentKey, StakeKey};
 pub use metadata::{build_cip25_aux_data, CIP25_LABEL};
-pub use mint::{build_signed_mint, build_signed_mint_with_metadata, build_unsigned_mint, PolicySpec};
+pub use mint::{
+    build_signed_cip68_nft_mint, build_signed_mint, build_signed_mint_with_metadata,
+    build_unsigned_mint, PolicySpec,
+};
+pub use sign::add_witness;
 pub use tx::{
     build_signed_payment, build_signed_payment_with_assets, build_unsigned_payment,
     build_unsigned_payment_with_assets, hex_decode, AssetSpec, InputUtxo, PaymentSummary,