Document the first run's time and logs; say why the VMs use -vga std

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -65,7 +65,8 @@ RAM and 4 CPUs per VM (`SMOKETEST_MEMORY`, `SMOKETEST_CPUS`), and disk space in
 `~/.cache/coredesktop-dev/smoketest`: VM disks grow as large as a real install,
 about 50 GB, and each run's VM disk is deleted when it's done.
 
-The first run builds two images, which takes a while:
+The first run builds two images before any scenario runs: base in a few
+minutes, installed in an hour or more. Later runs reuse them.
 
 - **base**: Arch's official cloud image, with GNOME added the way archinstall's
   desktop profile installs it. The user `tester` (password `tester`) logs in to
@@ -81,6 +82,15 @@ VM, so they download once. Flatpaks aren't cached.
 To watch a VM's screen, set `SMOKETEST_VNC=1` and connect a VNC viewer to
 `127.0.0.1:5901`.
 
+The terminal can stay quiet for many minutes while a VM boots or installs. To
+follow along, `tail -f` the logs in `~/.cache/coredesktop-dev/smoketest`:
+
+- `build-base/` and `build-installed/`, while an image builds: `build.log` is
+  what the build printed, and `serial.log` is the VM's console, the place to
+  look when a VM doesn't boot or doesn't answer on SSH.
+- `runs/<date>-<time>/<scenario>/`: `scenario.log` for each scenario, and
+  `serial.log` for its VM.
+
 ## Developing this project
 
 ```