Mempool: snapshotFromValidTxs rebuilds the GenTxId set on every snapshot

[`snapshotFromValidTxs`](https://github.com/IntersectMBO/ouroboros-consensus/blob/bd119d907b7701aac5f7825e6645614a7473e9bf/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Mempool/Impl/Common.hs#L510) builds `txIds = Set.fromList (map txId validTxs)` for `snapshotHasTx` on every snapshot.
`getSnapshot` calls it per invocation, and the tx-submission [mempool reader](https://github.com/IntersectMBO/ouroboros-consensus/blob/bd119d907b7701aac5f7825e6645614a7473e9bf/ouroboros-consensus-diffusion/src/ouroboros-consensus-diffusion/Ouroboros/Consensus/NodeKernel.hs#L903-L924) snapshots on every read, so the `O(n log n)` rebuild runs often.

The InternalState already maintains [`isTxIds :: Set (GenTxId blk)`](https://github.com/IntersectMBO/ouroboros-consensus/blob/bd119d907b7701aac5f7825e6645614a7473e9bf/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Mempool/Impl/Common.hs#L113) incrementally.
The `snapshotFromIS`/`getSnapshot` path could reuse it instead of rebuilding.
(The revalidation path `computeSnapshot` builds a fresh tx list, so it would still rebuild.)

Orthogonal to #2003: this is about how often the set is built, not the per-comparison cost.
Found while analysing #2003.

Start in ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Mempool/Impl/Common.hs at snapshotFromValidTxs, snapshotFromIS, getSnapshot, and InternalState.isTxIds. Trace the mempool reader in ouroboros-consensus-diffusion/.../NodeKernel.hs to understand the snapshot path. Done means the regular snapshot path reuses isTxIds for snapshotHasTx, while computeSnapshot still rebuilds its set.

Written by the indexing model from the issue text.