fix(fabricx): stop retaining RWSets and tx statuses in the vault

[SaidAltury-ibm]

closes #1599
The FabricX vault held two in-memory maps (rwsets, txStatuses) that grew
unboundedly: nothing on the FabricX path ever pruned or read them. FabricX has
no local commit pipeline — commit/validation happens in the external
fabric-x-committer, and finality is delivered by the gRPC notifier — so the
vault methods that populated or read those maps (CommitTX/DiscardTx/
SetDiscarded/Match/RWSExists) are never invoked in the FabricX wiring. Endorse-
only nodes were pinning ~1.5 GB of RWSets at rest as a result (PR #1206).

Changes:

• Remove the rwsets and txStatuses maps, the txStatusInfo struct, and the mutex.
  NewRWSet/NewRWSetFromBytes no longer retain the RWSet; the wrapper owns it.
• The five commit-pipeline interface methods now panic instead of returning a
  value. They exist only to satisfy fdriver.Vault; reaching one means the vault
  was wired into a generic committer by mistake, which is a programming error,
  not a runtime condition. Panicking fails loudly and cannot be silently
  swallowed — RWSExists in particular has no error channel, so its old false
  return was an actively misleading answer.
• Status now delegates to the batched Statuses so both report a tx unknown to
  the committer as Unknown ("not final yet") rather than as an error — the
  condition a caller polling an in-flight tx will hit.
• Bytes() passes namespace-version info to the marshaller per call instead of
  through a shared mutable field, fixing a data race between concurrent Bytes()
  calls. The marshaller is now stateless; the racy field and its exported
  wrapper are removed. Covered by a new -race test.

[github-actions]

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[mbrandenburger]

Thank you @SaidAltury-ibm - this is a solid PR. Please see my comments below.

[mbrandenburger]

Thank you @SaidAltury-ibm - greate updates on this PR. I think this PR is almost ready to be merged. Please see my minor comments below. Thank you

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