daemon split (1/5): extract *HostNetwork service

First phase of splitting the daemon god-struct into focused services
with explicit ownership.

HostNetwork now owns everything host-networking: the TAP interface
pool (initializeTapPool / ensureTapPool / acquireTap / releaseTap /
createTap), bridge + socket dir setup, firecracker process primitives
(find/resolve/kill/wait/ensureSocketAccess/sendCtrlAltDel), DM
snapshot lifecycle, NAT rule enforcement, guest DNS server lifecycle
+ routing setup, and the vsock-agent readiness probe. That's 7 files
whose receivers flipped from *Daemon to *HostNetwork, plus a new
host_network.go that declares the struct, its hostNetworkDeps, and
the factored firecracker + DNS helpers that used to live in vm.go.

Daemon gives up the tapPool and vmDNS fields entirely; they're now
HostNetwork's business. Construction goes through newHostNetwork in
Daemon.Open with an explicit dependency bag (runner, logger, config,
layout, closing). A lazy-init hostNet() helper on Daemon supports
test literals that don't wire net explicitly — production always
populates it eagerly.

Signature tightenings where the old receiver reached into VM-service
state:
 - ensureNAT(ctx, vm, enable) → ensureNAT(ctx, guestIP, tap, enable).
   Callers resolve tap from the handle cache themselves.
 - initializeTapPool(ctx) → initializeTapPool(usedTaps []string).
   Daemon.Open enumerates VMs, collects taps from handles, hands the
   slice in.

rebuildDNS stays on *Daemon as the orchestrator — it filters by
vm-alive (a VMService concern handles will move to in phase 4) then
calls HostNetwork.replaceDNS with the already-filtered map.

Capability hooks continue to take *Daemon; they now use it as a
facade to reach services (d.net.ensureNAT, d.hostNet().*). Planned
CapabilityHost interface extraction is orthogonal, left for later.

Tests: dns_routing_test.go + fastpath_test.go + nat_test.go +
snapshot_test.go + open_close_test.go were touched to construct
HostNetwork literals where they exercise its methods directly, or
route through d.hostNet() where they exercise the Daemon entry
points.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/daemon/fastpath_test.go

@@ -75,18 +75,18 @@ func TestTapPoolWarmsAndReusesIdleTap(t *testing.T) {
 		closing: make(chan struct{}),
 	}
 
-	d.ensureTapPool(context.Background())
-	tapName, err := d.acquireTap(context.Background(), "tap-fallback")
+	d.hostNet().ensureTapPool(context.Background())
+	tapName, err := d.hostNet().acquireTap(context.Background(), "tap-fallback")
 	if err != nil {
 		t.Fatalf("acquireTap: %v", err)
 	}
 	if tapName != "tap-pool-0" {
 		t.Fatalf("tapName = %q, want tap-pool-0", tapName)
 	}
-	if err := d.releaseTap(context.Background(), tapName); err != nil {
+	if err := d.hostNet().releaseTap(context.Background(), tapName); err != nil {
 		t.Fatalf("releaseTap: %v", err)
 	}
-	tapName, err = d.acquireTap(context.Background(), "tap-fallback")
+	tapName, err = d.hostNet().acquireTap(context.Background(), "tap-fallback")
 	if err != nil {
 		t.Fatalf("acquireTap second time: %v", err)
 	}