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>

kernel/core/console.c

@@ -170,6 +170,13 @@ cprintf(char *fmt, ...)
   if (locking)
     release(&cons.lock);
   va_end(ap);
+
+  // The kernel's own output reaches the second display too, and for the same
+  // reason it is done here rather than in consputc(): outside the lock. On a
+  // quiet boot almost nothing comes through this path -- kinfo() goes to the
+  // log -- so this is mostly for a verbose boot, where it is the whole point.
+  if (locking)
+    display_mirror_console();
 }
 
 // A line for the log, and for the screen only on a verbose boot.
@@ -753,6 +760,19 @@ consolewrite(struct inode *ip, uint64 off, char *buf, int n)
   for(i = 0; i < n; i++)
     vt_out(v, buf[i] & 0xff);
   release(&cons.lock);
+
+  // And onto a second display, if the kernel drives one.
+  //
+  // **After the lock, not inside it.** A display driver may have something to
+  // say -- vga_disp_acquire() announces its back buffer, the virtio driver
+  // complains when the device does not answer -- and cprintf() from inside
+  // this lock is a CPU acquiring a lock it already holds, which is a panic
+  // with a very confusing call chain. Found exactly that way.
+  //
+  // Once per write() rather than once per character, which is also the better
+  // batching: a line of output is one transfer instead of eighty.
+  display_mirror_console();
+
   ilock(ip);
 
   return n;