[Critical] DecisionHandler.on_decided() never called in HostMsg::Decided handler

[qj0r9j0vc2]

Problem

In crates/consensus/src/host.rs, the HostMsg::Decided handler does NOT call self.decision_handler.on_decided(). This means decided Cuts are never persisted, forwarded, or processed.

Location

crates/consensus/src/host.rs:268-316

Code Analysis

HostMsg::Decided {
    certificate,
    extensions: _,
    reply_to,
} => {
    let height = certificate.height;
    let round = certificate.round;
    
    info!(parent: &self.span, "Consensus decided on value");
    
    // Check for epoch transition
    let _epoch_transition = match self.on_block_committed(height) {
        // ...
    };
    
    // Reply with next height to start
    let next_height = height.next();
    // ... sends Next::Start ...
    
    // BUG: decision_handler.on_decided() is NEVER CALLED!
}

The DecisionHandler trait expects:

async fn on_decided(
    &self,
    height: ConsensusHeight,
    round: ConsensusRound,
    value: ConsensusValue,  // <-- This is needed
    certificate: CommitCertificate<CipherBftContext>,
) -> Result<(), ConsensusError>

Root Cause

Two issues compound this bug:

1. Handler never called: The HostMsg::Decided handler simply logs and replies with Next::Start without invoking decision_handler.on_decided()

2. Missing value lookup: Even if the handler tried to call on_decided(), it only has certificate.value_id - not the actual ConsensusValue. The value needs to be looked up from ChannelValueBuilder.cuts_by_value_id.

Impact

• Decided Cuts are lost: The backward-compatible spawn_host() sends Cuts to decided_tx, but that channel never receives anything
• Node loop stalls: In node.rs:run(), the decided_rx.recv() branch never triggers
• No Cut execution: The execution layer never receives decided Cuts
• State never advances: Block state root is never updated

Suggested Fix

HostMsg::Decided {
    certificate,
    extensions: _,
    reply_to,
} => {
    let height = certificate.height;
    let round = certificate.round;
    let value_id = certificate.value_id.clone();
    
    info!(parent: &self.span, height = height.0, "Consensus decided on value");
    
    // Look up the actual value from value_id
    // This requires the value_builder to expose a method to get value by id
    // or we need to store proposed values by value_id in the Host
    
    // Call decision handler
    if let Err(e) = self.decision_handler.on_decided(
        height,
        round,
        value,  // Need to obtain this from value_id lookup
        certificate.clone(),
    ).await {
        error!(parent: &self.span, error = %e, "Failed to process decided value");
    }
    
    // Check for epoch transition
    // ...
}

The ChannelValueBuilder already stores cuts by value_id in cuts_by_value_id. A getter method should be added to retrieve the value for the Decided handler.

Severity

Critical - This breaks the entire consensus finalization flow.