Simplify tx watch ready-queue lookup and log decode failures

[n13]

Summary

• Remove the fallback scan over the ready queue in TxWatch; rely solely on pool.ready_transaction() for lookup
• Add warn-level logging when stripping the length prefix or decoding a ready-queue transaction fails, instead of silently skipping

[n13]

Review

Verdict: LGTM (evaluation only, not a formal approval)

What I checked

• The removed fallback was dead code. The node pins TransactionPoolType::SingleState (node/src/service.rs:625). In the single-state pool, ready_transaction() is a direct hash lookup into the validated pool's ready set (validated_pool().ready_by_hash), and ready() iterates that same set — so the linear scan could never find a transaction the direct lookup missed. Even under the fork-aware pool, both methods read the same most-recent view (view_store.ready() / view_store.ready_transaction()), so the conclusion holds regardless of pool type.
• Warn-level logs on decode failure replace silent continues — decode failures here would indicate a real bug (a ready-pool extrinsic that doesn't decode as UncheckedExtrinsic), so surfacing them is right. The not-in-ready-queue case correctly stays at trace, since that's an expected race with block inclusion.
• Local verification: branch builds and all 17 txwatch tests pass.

Nit (non-blocking)

• Commit message is "minor tx watch update" — the PR title is the better description; worth squash-merging with the title.
• Build & Test Matrix was still running at review time; format and clippy are green.