…emove the schedule-time ValidateAndPrepare purge

The state cache carried a per-domain blockHash and was scrubbed by ValidateAndPrepare
before every block. In the parallel executor that call sits in processRequest — a
*schedule* step, not an apply step — copied from the serial path (#18868). With a
32-deep pipeline and heavy retry traffic the single blockHash almost never equals the
next call's parentHash, so it took the wipe branch ~100% of the time: measured
storage-cache purge_rate ~100%, hit ~35% during catch-up, the cache wiped every block.

Make the cache what it should be — a SharedDomains implementation detail, populated only
at flush (committed, fork-agnostic state) and invalidated only on unwind. Coherence is
now txNum/epoch based, no block awareness and no diffset:

- Each GenericCache entry carries (txNum, epoch). A read is valid iff it was written in
  the current epoch OR its txNum is at/below unwindFloor (predates every unwind).
- Unwind(txNum) bumps the epoch and lowers the floor — O(1), no scan. Stale entries are
  dropped lazily on their next read. txNum slots are reused across forks, so the epoch
  (not the txNum) tells a dead fork's write from the live fork's at the same txNum.
- CodeCache clears its small mutable addr layers on unwind; immutable content-addressed
  code is kept.

Wiring: FlushWithCallback delivers txNum (cache stamps it; branchCache derives step);
read-population and read-ahead stamp the step's txNum upper bound. The three exec-flow
ValidateAndPrepare calls are removed; the unwind path calls stateCache.Unwind(txNum)
unconditionally (diffset-free, matching the overlay's maxtx prune — diffsets aren't
generated below the reorg window, so the old changeSet-gated cache revert left a stale
gap). RevertWithDiffset/blockHash/ClearWithHash and the fork-validation cache scrub are
removed. (DB-level diffset retirement is a follow-on.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…emove the schedule-time ValidateAndPrepare purge

The state cache carried a per-domain blockHash and was scrubbed by ValidateAndPrepare
before every block. In the parallel executor that call sits in processRequest — a
*schedule* step, not an apply step — copied from the serial path (#18868). With a
32-deep pipeline and heavy retry traffic the single blockHash almost never equals the
next call's parentHash, so it took the wipe branch ~100% of the time: measured
storage-cache purge_rate ~100%, hit ~35% during catch-up, the cache wiped every block.

Make the cache what it should be — a SharedDomains implementation detail, populated only
at flush (committed, fork-agnostic state) and invalidated only on unwind. Coherence is
now txNum/epoch based, no block awareness and no diffset:

- Each GenericCache entry carries (txNum, epoch). A read is valid iff it was written in
  the current epoch OR its txNum is at/below unwindFloor (predates every unwind).
- Unwind(txNum) bumps the epoch and lowers the floor — O(1), no scan. Stale entries are
  dropped lazily on their next read. txNum slots are reused across forks, so the epoch
  (not the txNum) tells a dead fork's write from the live fork's at the same txNum.
- CodeCache clears its small mutable addr layers on unwind; immutable content-addressed
  code is kept.

Wiring: FlushWithCallback delivers txNum (cache stamps it; branchCache derives step);
read-population and read-ahead stamp the step's txNum upper bound. The three exec-flow
ValidateAndPrepare calls are removed; the unwind path calls stateCache.Unwind(txNum)
unconditionally (diffset-free, matching the overlay's maxtx prune — diffsets aren't
generated below the reorg window, so the old changeSet-gated cache revert left a stale
gap). RevertWithDiffset/blockHash/ClearWithHash and the fork-validation cache scrub are
removed. (DB-level diffset retirement is a follow-on.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>