(cherry picked from commit 856b696)

(cherry picked from commit 831742f)

…ment rebuild and CommitmentRoot

Replace LimitedHistoryStateReader (which fell back to GetLatest on miss and
silently leaked post-limit values) with a strict files-only reader for the
two callers that need a fixed boundary snapshot:

- RebuildCommitmentFiles uses FilesOnlyStateReader(rwTx, lastTxnumInShard-1)
  so commitment.kv is rebuilt only from the matching accounts/storage/code
  .kv files at the shard boundary.
- CheckCommitmentRoot's checkCommitmentRootViaSd uses FilesOnlyStateReader
  at maxTxNum, and checkCommitmentRootViaRecompute switches from an
  account-domain-only touchHistoricalKeys to sd.TouchChangedKeysFromHistory
  (accounts + storage; code is covered transitively via account.codeHash).

getLatestFromFiles walkback is also fixed: the previous bound skipped files
ending before maxTxNum, which broke the walkback semantics and made
files-only reads return nil for any key whose latest write lives in an
older .kv. Now only files starting strictly after maxTxNum are skipped.

… stage_exec

…flag for seg retire