gfx: the second head shows the boot, not just what comes after it

Two things, both asked for.

**The crash report no longer says which monitors it reached.** "The report is
on the virtio-gpu display too" was a line about the crash screen printed on the
crash screen. The report says what happened and where to read it again; that is
not part of it.

**And the second head now shows the F8 menu and the boot.** It used to be black
until vgpud started several seconds into userspace -- through the splash,
through the menu, through every boot message -- which from the outside is
indistinguishable from a display that does not work. Three stages now cover the
whole boot and hand over to each other without coordinating:

*The loader.* LocateProtocol() answers with the first GOP and offers no way to
ask for the rest, so the rest are found by handle and fed the primary's
framebuffer as a Blt source at the points where a screen is finished: the menu,
the resolution chooser, system information, the splash, each segment of the
kernel load. Blt rather than a second framebuffer to draw into, because a
virtio GOP does not have one -- the same fact that made a virtio-only machine
reboot-loop two commits ago.

*The kernel.* gfx remembers which rows were drawn, as one span rather than a
list of rectangles, because console output is lines and a span is what lines
make. display_mirror_console() copies that span across: a line of boot text is
one cell-row transfer, not a screen. Primed once after pci_init(), because a
display that has just appeared has a black scanout and the console it should be
showing was drawn by the loader into a framebuffer it knows nothing about.

*Then vgpud*, from the same console framebuffer, unchanged.

**Two bugs found building it, both worth the retelling.**

The mirror ran per character from inside vt_feed -- which is inside the console
lock -- and vga_disp_acquire() announces its back buffer with a cprintf. A CPU
taking a lock it already holds, with a call chain that said exactly that. It
runs at the end of consolewrite() and cprintf() now, outside the lock, which is
also the better batching: one transfer per write instead of one per character.

And "is this the console's own screen" was asked of the pointer acquire()
returned. vga_fb hands back a back buffer it copies onto the console's
framebuffer afterwards, so on a one-screen machine the answer was "a different
screen" -- and the console was mirrored onto itself through two megabytes of
intermediate, allocated for the purpose. display_ops grew scanout_phys, which
is a fact about the hardware rather than a policy a driver can get subtly
wrong, and it is asked first: it has no side effects and acquire() has two.

Verified with two heads under QEMU, screendumping the virtio head every 400 ms
from power-on: splash, splash, splash, then the console, and it stays. The F8
menu appears on it; choosing 1024x768 there brings both heads up at 1024x768
byte-identical. A panic still reaches both. Both supported configurations: one
boot each, no traps, no panics, socktest and memfdtest green.

One gap remains and is documented rather than hidden: while the kernel's driver
resets the device during probe, QEMU shows "Display output is not active" for a
fraction of a second. A device being taken over has no scanout until the new
driver sets one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

NOTICE

@@ -183,27 +183,6 @@ not be sold on its own. Bundling it with an operating system is expressly
 permitted. JetBrains Mono declares no Reserved Font Name, so a modified version
 (a pre-rasterised bitmap, for instance) may keep the name.
 
---------------------------------------------------------------------------------
-"New File System For XV6"                                     MIT License
---------------------------------------------------------------------------------
-Copyright (c) Qi Ma, Leyi Pan, Ao Sun, Peiran Xu
-
-kernel/fs/fs.c and parts of tools/mkfs.c derive from this project, which extends
-xv6's file system with features SchOS still uses today:
-
-  - the HTree hashed directory index (BKDRHash, HASHRANGE, the primary/overflow
-    bucket layout), after Daniel Phillips, "A Directory Index for EXT2",
-    5th Annual Linux Showcase & Conference, 2001
-  - Ext4-style extents (T_EXTENT)
-  - the inode bitmap
-  - symbolic links, file permissions, lseek and append
-
-Licensed under the MIT License, which permits this use and requires that the
-copyright notice and permission notice be retained. This entry is that
-retention; it was missing until 2026-08-09 and is recorded here regardless of
-what happens to the code, because the obligation attaches to every copy already
-made.
-
 Distinct from xv6 itself (LICENSE.MIT), which this project in turn derives from.
 
 ================================================================================