manager: tell apart missing transactions from mempool transactions

[bennyhodl]

Summary

Redo of #169. Blockchain::get_transaction_confirmations returned Ok(0) both for a transaction in the mempool and for a transaction the network does not know. With NB_CONFIRMATIONS=0 the manager moved contracts through states on transactions that were evicted from the mempool or never broadcast. The trait now returns a ConfirmationStatus enum — NotFound / InMempool / Confirmed(n) — and the manager handles each case. There is no recovery scan over closed contracts: the state machine no longer enters Closed on a transaction that is not on-chain, so there is nothing to recover.

Changes

• ddk-manager/src/lib.rs: new ConfirmationStatus enum with a confirmations() helper. Blockchain::get_transaction_confirmations returns it instead of a bare u32. This is a breaking change for trait implementers.
• ddk/src/chain/esplora.rs: when the status endpoint reports a transaction unconfirmed, query the transaction itself to tell "in the mempool" apart from "not found" (esplora reports both as unconfirmed). Transport errors stay Err, so a flaky esplora is not mistaken for a dropped transaction. The tip-height call now only runs for confirmed transactions.
• check_signed_contract: a missing funding transaction does not confirm the contract.
• check_for_spliced_contract: a missing splice funding transaction does not pre-close the previous contract.
• Pending close transactions that are missing are skipped instead of counted as zero confirmations.
• check_preclosed_contract: a missing closing transaction is broadcast again (safe when the chain source lags — the network accepts a transaction it already knows). If it cannot enter the mempool and the pre-close has no attestations (splice or cooperative close), the funding output is unspent and the contract moves back to Confirmed.
• Entering the terminal Closed state now requires at least one confirmation on-chain, also when NB_CONFIRMATIONS=0. No periodic check examines Closed contracts again, so a mempool-only transaction that later gets evicted must not put a contract there. This closes the exact incident path: with NB_CONFIRMATIONS=0 a splice contract now stays PreClosed until its funding transaction is mined, and reverts to Confirmed if that transaction is dropped.
• New integration test ddk/tests/chain.rs asserts the esplora client reports NotFound, InMempool, and Confirmed correctly against a regtest backend.

Testing

• cargo test -p ddk -p ddk-manager (full non-ignored suites) pass.
• NB_CONFIRMATIONS=6 cargo test -p ddk-manager --test manager_execution_tests -- --ignored --exact splice_in_enum_single_oracle_test cooperative_close_single_oracle_test enum_single_oracle_refund_test pass.
• Note: cooperative_close_single_oracle_test fails on master too when NB_CONFIRMATIONS is unset locally (CI sets it to 6); not related to this change.
• cargo clippy clean for the changed code; cargo check --workspace passes.