fix: add Parent-QC consistency check to prevent malformed blocks

[SherlockShemol]

Summary

Add validation that Block.Parent() == Block.QuorumCert().BlockHash() in Voter.Verify() to prevent Byzantine leaders from creating malformed blocks.

The Bug

The protocol was missing a fundamental check: Block.Parent must equal Block.QC.BlockHash.

Without this check, a Byzantine leader can create a malformed block where:

• Block.Parent points to an early/fork block
• Block.QC points to the latest valid block (existing QC, no need to forge)

Attack Scenario

Honest chain: Genesis → A(V1) → B(V2) → C(V3) → D(V4) → E(V5)

Byzantine leader (View 6) creates MALFORMED block F:
  • F.Parent = A.Hash     ← Points to early block A, NOT E!
  • F.QC = QC(E)          ← Uses valid QC pointing to E (already exists!)
  • F.View = 6

Result: F.Parent ≠ F.QC.BlockHash
  Parent chain: F → A (skips B, C, D, E!)
  QC chain:     F.QC → E → D → C → B → A

Why Attack Works (Before Fix)

Step	Action	Why It Works
1	Byzantine creates malformed block	NewBlock(parent, qc, ...) allows any combination
2	Sends via network	BlockFromProto deserializes independently
3	Honest nodes verify	VoteRule Liveness: qcBlock.View > bLock.View ✓
4	Honest nodes vote!	Voter.Verify had no Parent-QC check

Impact Without Fix

1. Blockchain data structure inconsistency: Parent chain diverges from QC chain
2. blockchain.Extends() returns incorrect results: It traverses Parent chain
3. Liveness failure: Subsequent proposals inherit inconsistent state

The Fix

In protocol/consensus/voter.go, Verify() now checks:

if proposal.Block.Parent() != proposal.Block.QuorumCert().BlockHash() {
    return fmt.Errorf("block parent %s does not match QC block %s",
        proposal.Block.Parent().SmallString(),
        proposal.Block.QuorumCert().BlockHash().SmallString())
}

Test

twins/parent_qc_mismatch_test.go - Creates a malformed block and verifies Voter.Verify() correctly rejects it.

=== RUN   TestParentQCMismatch/ecdsa
    Voter.Verify correctly rejected malformed block: block parent vklOxXpi does not match QC block LHnEvFTW
--- PASS: TestParentQCMismatch (0.02s)

Fixes #283