fix(monitoring): use MemAvailable instead of MemTotal-MemFree for Linux guest/node memory

When the Proxmox API's meminfo/status payload omits Available, Buffers,
and Cached (common for QEMU guest-agent and node status responses), the
code derived 'available' from Free alone — producing used = Total-Free
which counts reclaimable page cache as used memory (e.g. 94% instead of
the correct 76%).

Guest path (deriveGuestMemInfoAvailable): return 0 when cache metrics
are completely missing and only Free is available, so resolveGuestStatusMemory
tries better sources: guest-agent /proc/meminfo file-read (returns
MemAvailable), RRD memavailable, or the linked host agent.

Node path (resolveNodeMemory): always try RRD memavailable when cache
metrics are missing, not only when effectiveAvailable == 0 — previously
a non-zero Free value blocked the RRD fallback.

Refs #1501

internal/monitoring/guest_memory_sources.go

@@ -73,6 +73,9 @@ func deriveGuestMemInfoAvailable(memInfo *proxmox.VMMemInfo, guestRaw *VMMemoryR
 				}
 			}
 		}
+		if missingCacheMetrics {
+			return 0, ""
+		}
 		return componentAvailable, "derived-free-buffers-cached"
 	default:
 		if availableFromUsed > 0 && missingCacheMetrics {

internal/monitoring/guest_memory_sources_test.go

@@ -6,6 +6,100 @@ import (
 	"github.com/rcourtman/pulse-go-rewrite/pkg/proxmox"
 )
 
+func TestDeriveGuestMemInfoAvailable(t *testing.T) {
+	kb := uint64(1024)
+	miB := uint64(1024 * 1024)
+	giB := uint64(1024 * 1024 * 1024)
+
+	tests := []struct {
+		name          string
+		memInfo       *proxmox.VMMemInfo
+		wantAvailable uint64
+		wantSource    string
+	}{
+		{
+			name: "uses available field when present",
+			memInfo: &proxmox.VMMemInfo{
+				Total:     8 * giB,
+				Used:      6 * giB,
+				Free:      2 * giB,
+				Available: 4 * giB,
+				Buffers:   500 * miB,
+				Cached:    1 * giB,
+			},
+			wantAvailable: 4 * giB,
+			wantSource:    "available-field",
+		},
+		{
+			name: "derives from free+buffers+cached when available missing",
+			memInfo: &proxmox.VMMemInfo{
+				Total:   8 * giB,
+				Used:    5 * giB,
+				Free:    2 * giB,
+				Buffers: 500 * miB,
+				Cached:  1 * giB,
+			},
+			wantAvailable: 2*giB + 500*miB + 1*giB,
+			wantSource:    "derived-free-buffers-cached",
+		},
+		{
+			name: "returns zero when only free is available (no cache metrics)",
+			memInfo: &proxmox.VMMemInfo{
+				Total: 7796964 * kb,
+				Used:  7352140 * kb,
+				Free:  444824 * kb,
+			},
+			wantAvailable: 0,
+			wantSource:    "",
+		},
+		{
+			name: "returns availableFromUsed when used excludes cache despite missing cache metrics",
+			memInfo: &proxmox.VMMemInfo{
+				Total: 8 * giB,
+				Used:  3 * giB,
+				Free:  1 * giB,
+			},
+			wantAvailable: 5 * giB,
+			wantSource:    "derived-total-minus-used",
+		},
+		{
+			name: "returns zero when only total and used with used equal to total minus free",
+			memInfo: &proxmox.VMMemInfo{
+				Total: 8 * giB,
+				Used:  7 * giB,
+			},
+			wantAvailable: 1 * giB,
+			wantSource:    "derived-total-minus-used",
+		},
+		{
+			name: "returns zero for empty meminfo",
+			memInfo: &proxmox.VMMemInfo{
+				Total: 8 * giB,
+			},
+			wantAvailable: 0,
+			wantSource:    "",
+		},
+		{
+			name:          "nil meminfo returns zero",
+			memInfo:       nil,
+			wantAvailable: 0,
+			wantSource:    "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotAvailable, gotSource := deriveGuestMemInfoAvailable(tt.memInfo, nil)
+			if gotAvailable != tt.wantAvailable {
+				t.Fatalf("available = %d, want %d", gotAvailable, tt.wantAvailable)
+			}
+			if gotSource != tt.wantSource {
+				t.Fatalf("source = %q, want %q", gotSource, tt.wantSource)
+			}
+		})
+	}
+}
+
 func TestSelectGuestLowTrustUsedMemory(t *testing.T) {
 	giB := uint64(1024 * 1024 * 1024)
 

internal/monitoring/node_memory_sources.go

@@ -69,7 +69,7 @@ func (m *Monitor) resolveNodeMemory(
 	usedRRDAvailableFallback := false
 	rrdMemUsedFallback := false
 
-	if effectiveAvailable == 0 && missingCacheMetrics {
+	if missingCacheMetrics {
 		if metrics, err := m.getNodeRRDMetrics(ctx, client, nodeName); err == nil {
 			haveRRDMetrics = true
 			rrdMetrics = metrics

internal/monitoring/node_memory_sources_test.go

@@ -64,6 +64,22 @@ func TestResolveNodeMemoryCharacterization(t *testing.T) {
 			wantUsed:      6 * gib,
 			wantRawSource: "rrd-memused",
 		},
+		{
+			name: "node status has only free so rrd memavailable is preferred over total-minus-free",
+			memory: &proxmox.MemoryStatus{
+				Total: 8 * gib,
+				Used:  7 * gib,
+				Free:  1 * gib,
+			},
+			rrdPoints: []proxmox.NodeRRDPoint{{
+				MemTotal:     floatPtr(float64(8 * gib)),
+				MemAvailable: floatPtr(float64(4 * gib)),
+			}},
+			wantSource:    "rrd-memavailable",
+			wantFallback:  "rrd-memavailable",
+			wantUsed:      4 * gib,
+			wantRawSource: "rrd-memavailable",
+		},
 	}
 
 	for _, tt := range tests {