feat: support chained transactions with a mempool UTXO overlay

[satran004]

Summary

• add a bounded, single-writer mempool UTXO overlay for arbitrary-depth chained transactions
• atomically enforce duplicate, conflicting-spend, transaction-count, byte and UTXO-index admission rules
• resolve regular, reference and collateral inputs from consumed, produced and canonical UTXO state in order
• retain selected transactions through forge-to-apply and expose produced outputs in the block-local overlay
• reconcile confirmation, canonical competing spends, rollback and descendant invalidation without orphaning confirmed-parent children
• isolate canonical UTXO acknowledgements from derived-listener failures and require synchronous admission-critical listeners
• support reference scripts produced by unconfirmed parents during phase-2 validation
• add ADR-NET-009 and the deferred generalized-ledger-state ADR

Design notes

• Overlay indexes, dependency maps and locking remain internal to DefaultMemPool.
• Stateful validation is deliberately serialized for the initial correctness model.
• Selected entries remain in the mempool until canonical UTXO apply acknowledgement.
• ADR-NET-009 remains Proposed pending its retained-heap profiling acceptance gate.
• Certificate, governance and other non-UTXO mempool dependencies remain future work.

Verification

• ./gradlew :ledger-rules:test :runtime:test
• focused admission, chaining, conflicting-canonical-spend, reference-script and acknowledgement-isolation regression tests
• ./gradlew compileJava compileTestJava
• git diff --check

All completed successfully.

Closes #66

Deferred hardening and protocol-aware bounds are tracked in #68.