fix: disque via statvfs() — valeurs identiques à df

Remplace sysinfo::Disks par un appel direct à libc::statvfs("/").
- used  = (f_blocks − f_bfree) × f_frsize  → correspond à df "Utilisé"
- free  = f_bavail × f_frsize              → correspond à df "Dispo"
- total = f_blocks × f_frsize

Avant (sysinfo) : used comptait les blocs réservés root → surestimation de ~3-4 Go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent/src/main.rs

@@ -1,5 +1,5 @@
 use nanometrics_agent::{config, metrics, payload, transport};
-use sysinfo::{Components, Disks, Networks, System};
+use sysinfo::{Components, Networks, System};
 use std::time::{Duration, Instant};
 use std::sync::mpsc;
 
@@ -34,7 +34,6 @@ fn main() {
 
     let mut sys = System::new();
     let mut networks = Networks::new_with_refreshed_list();
-    let mut disks = Disks::new_with_refreshed_list();
     let mut components = Components::new_with_refreshed_list();
 
     let udp_sender = if cfg.protocols.udp.enabled {
@@ -102,9 +101,8 @@ fn main() {
         }
 
         if first_slow || now.duration_since(last_slow).as_secs() >= 60 {
-            disks.refresh();
             if cfg.metrics.disk.udp || cfg.metrics.disk.mqtt {
-                let (used, free, total) = metrics::disk::get(&disks);
+                let (used, free, total) = metrics::disk::get();
                 m.hdd_used = Some(used);
                 m.hdd_free = Some(free);
                 m.hdd_total = Some(total);