Simplex reconfiguration framework - Part V (ICM epoch + authInfo)

[yacovm]

Re-introduce ICM epoch info
Persist ICM epoch information in block metadata and maintain it on the
StateMachine the same way the proposerVM does, so blocks encode which
P-chain height the ICM protocol should use.

Metadata / encoding:

• Add ICMEpochInfo (canoto field 7) and AuxiliaryInfo (field 6) to the
  block metadata, with Equal/IsZero helpers.

ICM epoch computation:

• Add ICMEpoch, ICMEpochInput and the ICMEpochTransition function type,
  exposed via Config.ComputeICMEpoch (replacing GetUpgrades).
• Add computeICMEpochInfo, which derives the next epoch from the parent
  block's epoch, P-chain height and timestamps.
• Build/verify of normal, collecting-approvals and sealing blocks now
  compute and carry the ICM epoch, and feed its P-chain height into
  BlockBuilder.BuildBlock and InnerBlock.Verify (removing the prior
  TODOs and the hard-coded pChainHeight).

Zero block:

• The zero block carries over its parent's ICM epoch unchanged (empty
  when the parent predates ICM); the first epoch then begins on the
  block built on top of it.
• verifyBlockZero now validates the whole block by digest instead of
  comparing SimplexEpochInfo alone (bug fix!)

Timestamps:

• Add verifyTimestamp (rejects non-monotonic timestamps, timestamps too
  far in the future beyond maxSkew, and values exceeding int64) and
  apply it when verifying both zero and non-zero blocks.