feat: surface ZFS ARC reclaimable as dashboard context line (#1760)

* feat: surface ZFS ARC reclaimable as dashboard context line

Add arcReclaimable to the HostMemory interface and MemoryWire shape so
the reclaimable ARC amount computed by readReclaimableArc() is exposed
through /api/system/stats and /api/fleet/overview. Show it as a context
line on the dashboard memory tile, matching the balloon pattern.

ARC continues to feed the gauge percentage as before; this is a
display-only addition for operator visibility.

* docs: clarify ARC line requires nonzero reclaimable, not just readable stats

frontend/src/components/FleetView/types.ts

@@ -15,6 +15,7 @@ export interface FleetNodeSystemStats {
         used: number;
         free: number;
         usagePercent: string;
+        arcReclaimable?: number;
         ballooned?: number;
         effectiveTotal?: number;
         effectiveUsed?: number;

frontend/src/components/dashboard/ResourceGauges.tsx

@@ -116,6 +116,11 @@ export function ResourceGauges({ systemStats, cpuHistory, netHistory, historyEnd
             {ramEffectivePercent !== null ? ` (effective ${parseFloat(ramEffectivePercent).toFixed(0)}%)` : ''}
           </div>
         ) : null}
+        {systemStats?.memory.arcReclaimable && systemStats.memory.arcReclaimable > 0 ? (
+          <div className="mt-1 font-mono text-[10px] text-stat-subtitle/70">
+            ZFS ARC reclaimable: {formatBytes(systemStats.memory.arcReclaimable)}
+          </div>
+        ) : null}
         {systemStats ? <GaugeBar value={ramVal} /> : null}
       </div>
 

frontend/src/components/dashboard/types.ts

@@ -16,6 +16,7 @@ export interface SystemStats {
     used: number;
     free: number;
     usagePercent: string;
+    arcReclaimable?: number;
     ballooned?: number;
     effectiveTotal?: number;
     effectiveUsed?: number;