windows probe-only agent + agent version display unification + reality sni from sing-box fragment

Scope B (Windows agent):
- probe.go split into shared core + probe_linux.go (verbatim Linux
  sampler, build-tagged) + probe_windows.go (Win32 sampler: GetSystemTimes
  CPU diff, GlobalMemoryStatusEx, fixed-drive df totals, GetIfTable2
  counted set, MachineGuid boot identity, registry facts; load and
  TCP/UDP counts stay zero - x/sys v0.47.0 exports no wrapper)
- update intents are acknowledged and skipped on Windows (the release
  assets are Linux binaries; Windows nodes update via install-agent.ps1)
- cmd/xui-agent-windows: SCM service host (svc.Run + mgr install/
  uninstall/start/stop) mirroring main.go's agent lifecycle wiring
- install-agent.ps1: downloads the exe, writes probeOnly agent.json,
  registers and starts the service, rerun-safe

Scope C (version display):
- config.GetAgentDisplayVersion (and the master getter) compose from
  the bare number so a V-form injection no longer doubles the letter
  (AV2.1.0 -> A2.1.0)
- node list view repairs stored AV-prefixed reports at assembly so
  deployed agents read uniformly without an update

Scope D (production fix):
- share-link reality sni: when the inbound has a sing-box fragment,
  sni is the fragment's tls.server_name - the only name the sing-box
  REALITY server serves. The previous rule picked a uniformly random
  serverNames entry per generation, which xray tolerated but sing-box
  refuses as an invalid connection. sid keeps its random pick: the
  converter carries the whole shortIds list into the fragment, so
  every listed sid is served.

internal/agent/update.go

@@ -52,6 +52,15 @@ var execSelf = func(path string) error {
 	return syscall.Exec(path, append([]string{path}, os.Args[1:]...), os.Environ())
 }
 
+// selfUpdateSupported reports whether this platform can consume a
+// master update intent. Windows agents are the standalone
+// xui-agent-windows binary (see cmd/xui-agent-windows), serviced by
+// re-running install-agent.ps1: the release assets an intent points
+// at are the panel's Linux binaries, so an intent delivered to a
+// Windows node is acknowledged and skipped, never downloaded.
+// A package variable so tests can exercise the skip on any OS.
+var selfUpdateSupported = runtime.GOOS != "windows"
+
 func (a *Agent) setUpdateReport(st agentUpdateStatus) {
 	a.updateMu.Lock()
 	cp := st
@@ -82,6 +91,19 @@ func (a *Agent) maybeHandleUpdate(intent *updateIntent) {
 		a.updateMu.Unlock()
 		return
 	}
+	if !selfUpdateSupported {
+		// Windows (see selfUpdateSupported): log once per intent and
+		// mark it handled so the poll does not re-log it every cycle.
+		// No report is filed — the master-side intent simply ages out,
+		// and the node updates by re-running install-agent.ps1.
+		a.updateMu.Lock()
+		if intent.RequestedAt != a.updateHandled {
+			a.updateHandled = intent.RequestedAt
+			logger.Infof("agent update to %s offered but skipped on %s: Windows agents update by re-running install-agent.ps1", intent.Target, runtime.GOOS)
+		}
+		a.updateMu.Unlock()
+		return
+	}
 	a.updateMu.Lock()
 	if a.updateRunning || intent.RequestedAt == a.updateHandled ||
 		(intent.RequestedAt == a.updateFailedReq && time.Since(a.updateFailedAt) < updateRetryBackoff) {