fix(virtdisplay): keep 1x1x24 as the headless="virtual" default

Reverts the default half of 4b20b77. That commit raised Xvfb's root window
from 1x1x24 to 1920x1080x24 for #458, on the reasoning that a 1x1 root
"breaks anything that measures the screen".

That reasoning does not hold here:

- screen.* never comes from the root window. It comes from the generated
  fingerprint, applied per context in the browser, and
  clamp_screen_to_display() is skipped outright for virtual displays (the
  `not virtual_display` guard in utils.py), so a 1x1 root cannot clamp a
  generated screen down to 1x1.
- #458's actual symptom -- blank/dark screenshots -- does not reproduce on
  152.0.4-beta.28. Measured at both geometries on the same build, same page:

    Xvfb 1x1x24        493 distinct colours, 57.5% dominant  -> renders
    Xvfb 1920x1080x24  493 distinct colours, 55.8% dominant  -> renders

  Identical. Firefox composites offscreen, so the root window size does not
  gate rendering. A full-page screenshot of example.com under 1x1x24 is
  pixel-correct.

1x1x24 is Camoufox's long-standing default and has run that way for years.
CAMOUFOX_VIRTUAL_DISPLAY_SIZE is kept as an escape hatch for anyone who does
want a real framebuffer, and still validates its input.

The Composite half of 4b20b77 was already reverted separately in 75d09a3.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pythonlib/camoufox/virtdisplay.py

@@ -18,15 +18,17 @@ DISPLAYFD_READ_TIMEOUT_S = 10.0
 
 # Xvfb screen geometry for headless="virtual".
 #
-# This used to be hardcoded to 1x1x24. A 1x1 root window is not a plausible
-# desktop: it breaks anything that measures the screen, and it is why
-# clamp_screen_to_display() has to special-case virtual displays (otherwise a
-# generated fingerprint gets clamped to 1x1). Default to an ordinary desktop
-# size instead. The framebuffer cost is trivial -- 1920*1080*4 bytes is ~8MB.
+# 1x1x24 is Camoufox's long-standing default and stays the default. The root
+# window size is not observable as a fingerprint: screen.* comes from the
+# generated fingerprint, applied per context in the browser, and
+# clamp_screen_to_display() is skipped entirely for virtual displays (see the
+# `not virtual_display` guard in utils.py), so a 1x1 root never clamps a
+# generated screen down to 1x1.
 #
 # Override with CAMOUFOX_VIRTUAL_DISPLAY_SIZE="<width>x<height>x<depth>", e.g.
-# "2560x1440x24". Depth may be omitted.
-DEFAULT_SCREEN = "1920x1080x24"
+# "1920x1080x24", for the cases that do want a real framebuffer to draw into.
+# Depth may be omitted.
+DEFAULT_SCREEN = "1x1x24"
 SCREEN_ENV_VAR = "CAMOUFOX_VIRTUAL_DISPLAY_SIZE"
 
 # The Composite extension, disabled by default (Xvfb's `-extension COMPOSITE`).