Implement hysteresis for metric status changes to prevent flapping

Add comprehensive hysteresis support to prevent status oscillation near
threshold boundaries while maintaining responsive alerting.

Key Features:
- HysteresisThresholds with configurable upper/lower limits
- StatusTracker for per-metric status history
- Default gaps: CPU load 10%, memory 5%, disk temp 5°C

Updated Components:
- CPU load collector (5-minute average with hysteresis)
- Memory usage collector (percentage-based thresholds)
- Disk temperature collector (SMART data monitoring)
- All collectors updated to support StatusTracker interface

Cache Interval Adjustments:
- Service status: 60s → 10s (faster response)
- Disk usage: 300s → 60s (more frequent checks)
- Backup status: 900s → 60s (quicker updates)
- SMART data: moved to 600s tier (10 minutes)

Architecture:
- Individual metric status calculation in collectors
- Centralized StatusTracker in MetricCollectionManager
- Status aggregation preserved in dashboard widgets

shared/src/protocol.rs

@@ -1,5 +1,5 @@
-use serde::{Deserialize, Serialize};
 use crate::metrics::Metric;
+use serde::{Deserialize, Serialize};
 
 /// Message sent from agent to dashboard via ZMQ
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -65,28 +65,28 @@ impl MessageEnvelope {
             payload: serde_json::to_vec(&message)?,
         })
     }
-    
+
     pub fn command(command: Command) -> Result<Self, crate::SharedError> {
         Ok(Self {
             message_type: MessageType::Command,
             payload: serde_json::to_vec(&command)?,
         })
     }
-    
+
     pub fn command_response(response: CommandResponse) -> Result<Self, crate::SharedError> {
         Ok(Self {
             message_type: MessageType::CommandResponse,
             payload: serde_json::to_vec(&response)?,
         })
     }
-    
+
     pub fn heartbeat() -> Result<Self, crate::SharedError> {
         Ok(Self {
             message_type: MessageType::Heartbeat,
             payload: Vec::new(),
         })
     }
-    
+
     pub fn decode_metrics(&self) -> Result<MetricMessage, crate::SharedError> {
         match self.message_type {
             MessageType::Metrics => Ok(serde_json::from_slice(&self.payload)?),
@@ -95,7 +95,7 @@ impl MessageEnvelope {
             }),
         }
     }
-    
+
     pub fn decode_command(&self) -> Result<Command, crate::SharedError> {
         match self.message_type {
             MessageType::Command => Ok(serde_json::from_slice(&self.payload)?),
@@ -104,7 +104,7 @@ impl MessageEnvelope {
             }),
         }
     }
-    
+
     pub fn decode_command_response(&self) -> Result<CommandResponse, crate::SharedError> {
         match self.message_type {
             MessageType::CommandResponse => Ok(serde_json::from_slice(&self.payload)?),
@@ -113,4 +113,4 @@ impl MessageEnvelope {
             }),
         }
     }
-}
+}