Adding Support For Non-Validators

[samliok]

Non-Validator Node

Adds a nonvalidator package implementing a node that follows a Simplex chain,
verifying and indexing finalized blocks, without participating in consensus (it never
votes, proposes, or signs). It tracks the chain by collecting blocks + finalizations from
validators and replicating any sequences it's missing.

NonValidator (non_validator.go)

Similarly to the epoch we have a HandleMessage function. However we only listen to these message types:

• Block messages (handleBlock) — accepts a block only if it's from the round's
  leader, in a known epoch, and within the sequence window. Blocks are buffered in
  incompleteSequences until the matching finalization arrives.
• Finalizations (handleFinalization) — verifies the QC against the epoch's validator
  set. Matches against any buffered block; finalizations too far ahead or for unknown
  epochs are forwarded to the replicator / trigger a sealing-block request.
• Replication responses (handleReplicationResponse) — validates each fetched quorum
  round, stores it in the replication state, then schedules the next in-order finalized
  block for verification and indexing.

Once a block and a verified finalization are held for a sequence, a verification task is
scheduled (scheduleNewFinalizedBlockTask) through a BlockDependencyManager, which
enforces that sequences are verified/indexed in order.

Epoch validation (epochs.go, epoch_replicator.go)

A non-validator must know each epoch's validator set to verify finalizations. Validator
sets are carried in sealing blocks (new SealingBlockInfo, see below) which form a
backward-linked hash chain across epochs.

• epochs map holds only epochs whose validator set has been validated. Bootstrapped from
  storage in newEpochs (handles genesis / pre-Simplex Epoch 0).
• A new epoch is validated when (maybeValidateNextEpoch):
  1. we receive/verify the sealing block normally, or
  2. canValidate — we already trust the next epoch and this block matches its
     prevSealingBlockHash (validate backward down the chain), or
  3. epochReplicator.collectedQuorumRound — we've collected F+1 matching sealing-block
     responses from distinct nodes for an epoch we can't otherwise verify

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