Implement BFT finality gadget with equivocation detection and slashing (#115)

* Initial plan

* Implement BFT finality gadget with equivocation detection and slashing

Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

* Add finality integration tests and comprehensive documentation

Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

* Fix security issues: overflow protection, proper error handling, and stake counting

Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

* Add implementation summary documentation for finality gadget

Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Steake <530040+Steake@users.noreply.github.com>

Author: Copilot

crates/bitcell-consensus/src/block.rs

@@ -1,6 +1,7 @@
 //! Block structures
 
 use bitcell_crypto::{Hash256, PublicKey, Signature};
+use crate::finality::{FinalityVote, FinalityStatus};
 use serde::{Deserialize, Serialize};
 
 /// Block header
@@ -58,6 +59,13 @@ pub struct Block {
 
     /// Proposer signature
     pub signature: Signature,
+    
+    /// Finality votes collected for this block
+    pub finality_votes: Vec<FinalityVote>,
+    
+    /// Finality status of this block
+    #[serde(default)]
+    pub finality_status: FinalityStatus,
 }
 
 impl Block {