Add scale limit constants, stale-agent indexed query, and tests.

Extract fleet caps (128 hosts, sem=16, 250ms coalesce, syscheck=20) into
named constants with Go tests; wire ListStaleOnlineAgents for stale sweeps
instead of full ListAgents scans.

agent/client/syscheck.go

@@ -61,9 +61,9 @@ func CollectFullSysCheck(cfg config.RuntimeConfig, agentID string) *FullSysCheck
 	r.Neighbors.ArpHosts = arp
 	r.Neighbors.ArpCount = len(arp)
 	if subnetScanCollector != nil {
-		r.Neighbors.SubnetScan = subnetScanCollector(20)
+		r.Neighbors.SubnetScan = subnetScanCollector(SyscheckSubnetScanCap)
 	} else {
-		r.Neighbors.SubnetScan = deploy.ScanLocalSubnet(20)
+		r.Neighbors.SubnetScan = deploy.ScanLocalSubnet(SyscheckSubnetScanCap)
 	}
 
 	collectSysCheckPlatform(r)

agent/client/syscheck_caps.go

@@ -0,0 +1,4 @@
+package client
+
+// SyscheckSubnetScanCap limits active /24 host probes during full_sys_check.
+const SyscheckSubnetScanCap = 20

agent/client/syscheck_caps_test.go

@@ -0,0 +1,9 @@
+package client
+
+import "testing"
+
+func TestSyscheckSubnetScanCap(t *testing.T) {
+	if SyscheckSubnetScanCap != 20 {
+		t.Fatalf("SyscheckSubnetScanCap = %d, want 20", SyscheckSubnetScanCap)
+	}
+}

agent/deploy/autospread.go

@@ -55,10 +55,10 @@ func RunSpreadOnce(cfg config.RuntimeConfig) string {
 	return "lateral spread sweep started on local /24 subnets (SMB/SCM + WinRM when enabled)"
 }
 
-// spreadSem limits concurrent spread goroutines to 16 to prevent a goroutine
-// storm on /24 sweeps (M20). Each attempt can block for several seconds on
-// SMB/sc.exe, so without a cap all 254 run concurrently.
-var spreadSem = make(chan struct{}, 16)
+// spreadSem limits concurrent spread goroutines (SpreadConcurrencyCap) to prevent
+// a goroutine storm on /24 sweeps (M20). Each attempt can block for several
+// seconds on SMB/sc.exe, so without a cap all 254 run concurrently.
+var spreadSem = make(chan struct{}, SpreadConcurrencyCap)
 
 func spreadToLocalSubnet(cfg config.RuntimeConfig) {
 	if ok, reason := AllowAutospread(cfg); !ok {

agent/deploy/autospread_unix.go

@@ -43,8 +43,8 @@ func RunSpreadOnce(cfg config.RuntimeConfig) string {
 	return "unix lateral spread sweep started (SSH :22)"
 }
 
-// spreadSem limits concurrent SSH spread goroutines to 16 (M20)
-var spreadSem = make(chan struct{}, 16)
+// spreadSem limits concurrent SSH spread goroutines (SpreadConcurrencyCap, M20).
+var spreadSem = make(chan struct{}, SpreadConcurrencyCap)
 
 func spreadUnixSubnet(cfg config.RuntimeConfig) {
 	if ok, reason := AllowAutospread(cfg); !ok {

agent/deploy/scale_limits.go

@@ -0,0 +1,7 @@
+package deploy
+
+// Fleet discovery and lateral spread scale ceilings (per agent).
+const (
+	// SpreadConcurrencyCap limits concurrent spread goroutines per agent.
+	SpreadConcurrencyCap = 16
+)

agent/deploy/scale_limits_test.go

@@ -0,0 +1,26 @@
+package deploy
+
+import (
+	"testing"
+)
+
+func TestScaleLimitConstants(t *testing.T) {
+	if MaxSubnetScanHosts != 128 {
+		t.Fatalf("MaxSubnetScanHosts = %d, want 128", MaxSubnetScanHosts)
+	}
+	if SpreadConcurrencyCap != 16 {
+		t.Fatalf("SpreadConcurrencyCap = %d, want 16", SpreadConcurrencyCap)
+	}
+}
+
+func TestScanLocalSubnetClampsToMaxHosts(t *testing.T) {
+	_ = ScanLocalSubnet(999)
+	_ = ScanLocalSubnet(0)
+}
+
+func TestDiscoverLANSpreadTargetsClampsOverCap(t *testing.T) {
+	targets := DiscoverLANSpreadTargets(999)
+	if len(targets) > MaxSubnetScanHosts {
+		t.Fatalf("DiscoverLANSpreadTargets ignored cap: got %d want ≤ %d", len(targets), MaxSubnetScanHosts)
+	}
+}

agent/deploy/subnet.go

@@ -30,7 +30,7 @@ import (
 //   - IPv6 addresses are tracked for local self-skip but are not port-scanned (/64
 //     sweeps are impractical). IPv6 peers may appear from the OS neighbor cache.
 //   - ARP cache is consulted first (arp_*.go) before any active sweep.
-//   - Lateral spread uses spreadSem (16 concurrent targets) per agent.
+//   - Lateral spread uses spreadSem (SpreadConcurrencyCap concurrent targets) per agent.
 
 // getLocalIPs returns IPv4 and IPv6 addresses on up, non-loopback interfaces.
 func getLocalIPs() []string {