tstest/natlab/vmtest: add web UI for watching VM tests live

Add an optional --vmtest-web flag that starts an HTTP server showing a
live dashboard for vmtest runs. The dashboard includes:

- Step progress tracker showing all test phases (compile, image prep,
  QEMU launch, agent connect, tailscale up, test-specific steps)
  with status icons and elapsed times
- Per-VM "virtual monitor" cards showing serial console output
  streamed in realtime via WebSocket
- Per-NIC DHCP status (supporting multi-homed VMs like subnet routers)
- Per-node Tailscale status (hidden for non-tailnet VMs)
- Test status badge (Running/Passed/Failed) with live elapsed timer
- Event log showing all lifecycle events chronologically

Architecture follows the existing util/eventbus HTMX+WebSocket pattern:
the server pushes HTML fragments with hx-swap-oob attributes over a
WebSocket, and HTMX routes them to the correct DOM elements by ID.

Key components:
- vmstatus.go: Step tracker (Begin/End lifecycle), EventBus (pub/sub
  with history for late joiners), VMEvent types, NodeStatus tracking
- web.go: HTTP server, WebSocket handler, template loading, ANSI-to-HTML
  conversion via robert-nix/ansihtml, deterministic port selection
- assets/: HTML templates, CSS, HTMX library (copied from eventbus)
- vnet/vnet.go: DHCP event callback on Server for observing DHCP lifecycle
- qemu.go: Console log file tailing with manual offset-based reading

Usage:
  go test ./tstest/natlab/vmtest/ --run-vm-tests --vmtest-web=:0 -v

When using :0, a deterministic port based on the test name is tried
first so re-runs get the same URL, falling back to OS-assigned on
conflict.

Updates #13038

Change-Id: I45281347b3d7af78ed9f4ff896033984f84dcb4d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

shell.nix

@@ -16,4 +16,4 @@
 ) {
   src =  ./.;
 }).shellNix
-# nix-direnv cache busting line: sha256-5uzkG6NQh0znjgE6yV5b01y8bUlTvLqXyAoWbMRQNEY=
+# nix-direnv cache busting line: sha256-ruRbOB2W9snyOYY0+6OD5IndI/JJKqrhTuPlBsKikRc=