Add Finality Gadget: BFT, Slashing, Rapid Confirmation

[Steake]

Part of Epic: #79 - RC3: Network Scalability & Production Infrastructure

---

Goal: Implement BFT finality for faster block confirmation.

Requirements:

• 2/3 stake agreement for finality
• Blocks irreversible after finality
• <1 minute finality time
• Double-sign slashing
• Evidence submission mechanism

Acceptance Criteria:

• Finalized blocks cannot be reverted
• Equivocation results in slashing
• Finality achieved consistently

See [RELEASE_REQUIREMENTS.md](https://github.com/Steake/BitCell/blob/master/docs/RELEASE_REQUIREMENTS. md)

[Steake]

[autopilot] @copilot please start work on this issue and open a PR that references #66 in the PR description.