phase 4.1-4.3: plutus script spend

new aldabra-core::plutus module:
- PlutusVersion enum (V1, V2, V3) → maps to ScriptKind on the
  pallas-txbuilder side.
- PlutusExUnits (mem, steps) — public mirror of pallas's so callers
  don't drag pallas types in. From<> impl converts internally.
- DEFAULT_EX_UNITS = (14M mem, 10B steps) — generous budget that
  validates trivial validators ("always succeeds", simple equality);
  real validators tune via the ex_units arg.
- MIN_COLLATERAL_LOVELACE = 5_000_000 (Conway protocol floor).
- build_signed_plutus_spend(payment, network, locked, script, redeemer,
  witness_datum?, available_utxos, change_addr, payout_addr,
  payout_lovelace, ex_units, params) → signed cbor.
  - picks the largest wallet UTXO ≥ 5 ADA as collateral, errors out
    if none qualifies.
  - happy path: locked + collateral as inputs, payout + change as
    outputs, script + redeemer + (optional witness) datum as
    witnesses, wallet's payment key signs the body.
  - reference inputs (4.2 expansion) and live ExUnits estimation
    (4.4) are follow-ups.
- looks_like_script_address(bech32) bool sanity helper for callers
  that want to filter by address kind before constructing a spend.

mcp tool wallet.script.spend: full args surface for one-shot
spend. plutus_version is a string ("v1"|"v2"|"v3"). ex_units optional.

84 → 88 unit tests. 15 → 16 mcp tools.

phase 4 status:
- 4.1 ☑ inline datum (already supported via Output::set_inline_datum
  used by cip-68 mint)
- 4.2 ◐ reference input (txbuilder has the API; not yet exposed in
  build_signed_plutus_spend — followup)
- 4.3 ☑ wallet.script.spend
- 4.4 ☐ ExUnits estimation — needs uplc / aiken integration, defer
- 4.5 ☑ stake key derivation
- 4.6 ☑ wallet.stake.delegate

crates/aldabra-core/src/lib.rs

@@ -39,6 +39,7 @@ pub mod cip68;
 pub mod derive;
 pub mod metadata;
 pub mod mint;
+pub mod plutus;
 pub mod sign;
 pub mod stake;
 pub mod tx;
@@ -53,6 +54,10 @@ pub use mint::{
     build_unsigned_mint, PolicySpec,
 };
 pub use sign::add_witness;
+pub use plutus::{
+    build_signed_plutus_spend, looks_like_script_address, PlutusExUnits, PlutusInput,
+    PlutusVersion, DEFAULT_EX_UNITS, MIN_COLLATERAL_LOVELACE,
+};
 pub use stake::{build_signed_stake_delegation, parse_pool_id, STAKE_KEY_DEPOSIT_LOVELACE};
 pub use tx::{
     build_signed_payment, build_signed_payment_with_assets, build_unsigned_payment,