[lisicky]

Builds on #862 (its commits are included unchanged, on the same base) and replaces its evaluator: "scalus" option with a plain IEvaluator default. Opened as an alternative to #862's API; the rest of #862 (network option, declared key inputs, known UTxOs for a local evaluator) is kept.

Why

evaluator?: IEvaluator | "scalus" makes the builder recognise one evaluator by identity: a private scalusEvaluator field, clone bookkeeping to keep that identity, and a validity-interval check that lives in the builder although it is about the evaluator's slot config. Mesh's default evaluator should be an ordinary IEvaluator instance, and whatever the builder knows that an evaluator could use should reach it through the interface, so any evaluator can take it.

What changes

• MeshTxBuilderOptions.evaluator is IEvaluator | null. Left out, with a fetcher, the builder evaluates locally with OfflineEvaluatorScalus, the default Mesh chooses (Scalus today). null turns evaluation off: redeemers keep the units declared on them. Without a fetcher there is no default, as there is nothing to resolve inputs with. MeshTxBuilderV2 inherits all of this.
• IEvaluator gains two optional members. acceptsKnownUtxos: a local evaluator sets it to receive every UTxO the builder holds (held inputs, declared key inputs), whether or not they are on chain yet. configure(context): takes an EvaluationContext with the network and the cost models the builder learned while completing. Remote providers implement neither and behave as before, with one improvement: an input given for evaluation is passed as its fetched copy when the builder has one, since a given copy may lack script bytes (utxosForEvaluation).
• OfflineEvaluatorScalus takes an optional network, implements configure, keeps a slot config given to its constructor whatever network it is told, and checks the validity interval of the serialized transaction itself, before any fetch. getTransactionValidityInterval is added to core-cst's transaction utils.
• MeshTxBuilder loses scalusEvaluator, configureScalusEvaluator and the "scalus" constructor branch. evaluateRedeemers calls evaluator.configure?.() and passes known UTxOs when acceptsKnownUtxos is set (utxosForScalusEvaluator is now utxosForLocalEvaluator). clone() passes evaluator ?? null, so a clone never makes a default of its own.

Behaviour changes to be aware of

• Local evaluation is on by default. Any new MeshTxBuilder({ fetcher }) that builds a transaction with a Plutus script witness now evaluates it with Scalus. Scalus loads on first evaluation (an ESM chunk, through a dynamic import()), and inputs the builder does not hold are fetched once per complete(). Under Jest this needs --experimental-vm-modules, which mesh-transaction, mesh-contract and mesh-scalus-emulator already run with. Five mesh-transaction tests that build Plutus transactions over an OfflineFetcher got evaluator: null to keep testing with declared units.
• The unknown-network error is now thrown by the evaluator, so it surfaces as Tx evaluation failed: The transaction has a validity interval, but the network it is for is unknown ..., like other evaluation errors. It is checked on the serialized transaction: invalidBefore(0), which the serializer leaves out, no longer needs a known network (feat(tx-builder): opt-in Scalus evaluation with evaluator: "scalus" #862's test for it is changed accordingly).
• A Scalus evaluator passed in is configured too, and gets the builder's UTxOs. One made for preprod but used with a mainnet change address and no network option follows the builder to mainnet; a slot config given to the constructor is never replaced. It no longer fetches inputs the builder already holds.
• mesh-x402's SpendContext doc comment is updated; its code already passes an evaluator explicitly.

Tests

• scalus-evaluator.test.ts (mesh-transaction): the default is Scalus in both builders, none without a fetcher, shared with clones, off with evaluator: null; a given evaluator is left alone, or configured if it takes a context, or given every held UTxO if it is local; the evaluator is kept across reset(); network inference and the unknown-network errors; slot 0.
• offline-evaluator-scalus.test.ts (core-cst): configure with a network, a pinned slot config and cost models version by version; the unknown-network error before any fetch; the TxEvaluator kept when nothing new is configured.
• test/utils/transaction.test.ts (core-cst): getTransactionValidityInterval, including a zero start.
• Two existing tests were adjusted because evaluation is now on by default: balance.test.ts "should balance even when outputs don't have lovelaces" (its fixture stores bare scripts in scriptRef, which Scalus cannot decode; it tests balancing, so evaluator: null) and provider-calls.test.ts "offline evaluator fetches each transaction once across evaluations" (a given OfflineEvaluatorScalus now gets the builder's UTxOs and fetches nothing itself, which the test now asserts, with the builder's fetcher asked once per transaction).
• provider-calls.test.ts "fetches missing script bytes for a locally known Plutus reference UTxO" fails on the base branch and on feat(tx-builder): opt-in Scalus evaluation with evaluator: "scalus" #862 (Script not found); utxosForEvaluation above fixes it.

Verification

Run locally after npm run build:mesh:

package	tests
mesh-core-cst	182 passed, plus 16 under the Scalus (ESM) config
mesh-transaction	197 passed
mesh-scalus-emulator, mesh-contract, mesh-x402, mesh-wallet	24, 1, 98, 34 passed

git merge-tree with #861: no conflicts. CI does not run for PRs into this branch.