feat: add SetMechanism sudo action + boot migration to mechanism_id=0

- New SudoAction::SetMechanism to change a challenge's mechanism_id via sudo
- CLI command: platform-sudo set-mechanism -c <id> -m <mechanism_id>
- Boot migration: forces all existing challenges to mechanism_id=0
  (fixes challenges stored with old default of 1)

Author: echobt

bins/platform-sudo/src/main.rs

@@ -73,6 +73,15 @@ enum Commands {
         #[arg(short, long)]
         weight: f64,
     },
+    /// Set a challenge's mechanism ID
+    SetMechanism {
+        /// Challenge ID (UUID or name)
+        #[arg(short = 'c', long)]
+        challenge_id: String,
+        /// Mechanism ID
+        #[arg(short, long)]
+        mechanism_id: u8,
+    },
     /// Emergency pause the network
     Pause {
         /// Reason for the pause
@@ -523,6 +532,19 @@ async fn main() -> Result<()> {
             });
             sudo_cli.send_sudo_action(action).await?;
         }
+        Some(Commands::SetMechanism {
+            challenge_id,
+            mechanism_id,
+        }) => {
+            let cid = ChallengeId::from_string(&challenge_id);
+            let action = serde_json::json!({
+                "SetMechanism": {
+                    "challenge_id": cid.0.to_string(),
+                    "mechanism_id": mechanism_id
+                }
+            });
+            sudo_cli.send_sudo_action(action).await?;
+        }
         Some(Commands::Pause { reason }) => {
             let action = serde_json::json!({
                 "EmergencyPause": {

bins/validator-node/src/main.rs

@@ -830,6 +830,25 @@ async fn main() -> Result<()> {
         info!("RPC server started on {}", args.rpc_addr);
     }
 
+    // Migrate: ensure all challenges use mechanism_id=0 (main mechanism)
+    {
+        let mut cs = chain_state.write();
+        let mut migrated = 0u32;
+        for (_id, cfg) in cs.wasm_challenge_configs.iter_mut() {
+            if cfg.config.mechanism_id != 0 {
+                info!(
+                    "Migrating challenge {} mechanism_id {} -> 0",
+                    cfg.name, cfg.config.mechanism_id
+                );
+                cfg.config.mechanism_id = 0;
+                migrated += 1;
+            }
+        }
+        if migrated > 0 {
+            info!("Migrated {} challenges to mechanism_id=0", migrated);
+        }
+    }
+
     // Reload WASM modules from persisted challenges on startup
     if let Some(ref executor) = wasm_executor {
         let challenges: Vec<(platform_core::ChallengeId, String)> = {

crates/core/src/message.rs

@@ -288,6 +288,12 @@ pub enum SudoAction {
         emission_weight: f64,
     },
 
+    /// Set a challenge's mechanism ID
+    SetMechanism {
+        challenge_id: ChallengeId,
+        mechanism_id: u8,
+    },
+
     // === Version Management ===
     /// Set required validator version (triggers auto-update)
     SetRequiredVersion {

crates/core/src/state.rs

@@ -366,6 +366,26 @@ impl ChainState {
                     )));
                 }
             }
+            SudoAction::SetMechanism {
+                challenge_id,
+                mechanism_id,
+            } => {
+                tracing::info!(
+                    challenge_id = %challenge_id,
+                    mechanism_id = mechanism_id,
+                    "Sudo: setting challenge mechanism ID"
+                );
+                if let Some(wasm_config) = self.wasm_challenge_configs.get_mut(challenge_id) {
+                    wasm_config.config.mechanism_id = *mechanism_id;
+                } else if let Some(challenge) = self.challenges.get_mut(challenge_id) {
+                    challenge.config.mechanism_id = *mechanism_id;
+                } else {
+                    return Err(crate::MiniChainError::Consensus(format!(
+                        "Challenge {} not found",
+                        challenge_id
+                    )));
+                }
+            }
             SudoAction::SetRequiredVersion {
                 min_version,
                 recommended_version,