feat(discovery): Windows local-discovery + local route pinning + prompt reconnect

Completes the mDNS local-discovery feature on Windows (was Linux-only since
v2.1.2). Three parts:

1. Windows hosts override (hosts_override_windows.go): %SystemRoot%...\\hosts,
   CRLF-aware read/write, ipconfig /flushdns after each change. The agent runs
   as a SYSTEM service so elevation is a non-issue. hosts_override.go split
   into shared rewrite logic + platform files; the hosts tests now run the real
   Windows write path on CI instead of skipping.

2. Local route pinning (local_route*.go): the hosts override only fixes name
   resolution -- the packet path is the routing table's job. If the WG mesh
   tunnel is up with AllowedIPs covering the LAN (or full-tunnel 0.0.0.0/0) it
   swallows the direct connection. Discovery now pins a /32 host route via the
   owning local interface (route.exe metric 1 on Windows, ip route replace on
   Linux) and drops it on revert. Closes a gap in the shipped Linux path too.

3. Prompt reconnect: apply/revert signals the WS loop so it reconnects
   immediately instead of waiting out the 5s backoff.

Route/hosts code is injectable + unit tested; go test passes natively on
Windows (this machine), and linux/amd64 + windows/arm64 cross-builds are clean.

local_route_unix.go

@@ -0,0 +1,36 @@
+//go:build !windows
+
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Unix host-route pinning via `ip route`. The discovered IP is on-link, so a
+// /32 route straight out the owning interface is enough; `ip route replace`
+// is idempotent (re-adds instead of failing when the route is already there).
+// The /32 wins by longest-prefix-match over any broader tunnel route, even a
+// full-tunnel 0.0.0.0/0 -- no metric games needed on Linux.
+
+func addHostRoute(ip string, _ int, ifaceName string) error {
+	out, err := routeExec("ip", "route", "replace", ip+"/32", "dev", ifaceName)
+	if err != nil {
+		return fmt.Errorf("ip route replace %s via %s: %v: %s", ip, ifaceName, err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
+func delHostRoute(ip string) error {
+	out, err := routeExec("ip", "route", "del", ip+"/32")
+	if err != nil {
+		// "No such process" / RTNETLINK errors mean the route isn't there;
+		// nothing to drop, not an error.
+		lower := strings.ToLower(string(out))
+		if strings.Contains(lower, "no such process") || strings.Contains(lower, "cannot find") {
+			return nil
+		}
+		return fmt.Errorf("ip route del %s: %v: %s", ip, err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}