panic: blink the keyboard lights, and mark the screen on the way in

A laptop has no serial port, and the machine we are trying to boot stops with
the loader's splash still on screen. Three completely different things look
exactly like that: the jump never landed, the kernel started and hung, or the
kernel started and panicked with a full report drawn into a framebuffer nobody
can see. They call for different next steps, so the kernel now says which
without needing a console at all.

**A bar, as the kernel's first act.** Eight lines of accent colour across the
top of the framebuffer, written straight through the handoff block before a
single subsystem exists -- no gfx, no scanout, no allocator, nothing that can
itself fail. If it appears over the splash, the jump landed and the kernel is
executing. If it does not, nothing below that point is worth looking at.
Skipped when the framebuffer is above the direct map, because a fault there
leaves the machine quieter still, and free on a boot that works: the console
clears the screen a few lines later.

**Blinking keyboard lights, on panic.** Scroll, Num and Caps together, about
three times a second, for ever. It is the one output every PC has had since
before any of this existed -- no driver, no interrupt, no memory -- and it
works when the graphics do not. Linux does the same thing for the same reason.

Every wait in it is bounded. It runs after `panicked` is set so there is
nothing left to protect, but a machine that hangs inside its own death rattle
tells you less than one that stops quietly. The delay is microdelay, which
spins on the TSC: no timer, no interrupt, no calibration -- all three of which
a kernel that died during boot may not have.

That makes the three cases distinguishable from across the room: no bar and no
blink is a jump that did not land; a bar and no blink is a kernel that hung; a
blinking keyboard is a panic, and the report is in /proc/kmsg after a restart.

**And the loader now describes the mode it is actually handing over**, not only
the one the firmware started in. On a machine where changing the resolution
loses the splash entirely, the difference between those two lines is the bug
report; on a boot that works nobody sees either, because ClearScreen runs a few
lines later.

Verified: `echo c > /proc/sysrq`, then QMP polled for five and a half seconds
afterwards -- the machine answers throughout and a screendump still shows the
panic report intact, so the blink loop runs without faulting, resetting, or
scribbling on what it is there to accompany. Both supported configurations
boot clean with the early bar in place.

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

kernel/core/main.c

@@ -50,6 +50,34 @@ kmain(uint32 fb_addr)
   boot_verbose = *(uchar*)P2V(SCHBOOT_VERBOSE_FLAG);
   boot_debug   = *(uchar*)P2V(SCHBOOT_DEBUG_FLAG);
 
+  // The kernel's first act: put a mark on the screen.
+  //
+  // Eight lines, before a single subsystem exists, because "the loader
+  // finished and the machine froze" has two completely different causes and
+  // nothing else here can tell them apart. If this bar appears over the
+  // loader's splash, the jump landed, the page tables are good and the kernel
+  // is executing; if it does not, none of those are true and everything below
+  // is the wrong place to look.
+  //
+  // It writes straight through the handoff block rather than through gfx,
+  // because gfx needs a scanout attached and attaching one is itself a thing
+  // that can fail. Skipped when the framebuffer is above the direct map -- a
+  // fault here would be a triple fault with the machine even quieter than it
+  // was. It costs nothing on a boot that works: the console clears the screen
+  // a few lines later.
+  {
+    uint64 pa = *(uint64*)P2V(SCHBOOT_FB_BASE);
+    uint32 w  = *(uint32*)P2V(SCHBOOT_FB_WIDTH);
+    uint32 st = *(uint32*)P2V(SCHBOOT_FB_STRIDE);
+
+    if (pa && w && st && pa + (uint64)st * 8 <= 4ULL << 30) {
+      volatile uint32 *px = (volatile uint32 *)P2V(pa);
+      for (uint32 y = 0; y < 8; y++)
+        for (uint32 x = 0; x < w; x++)
+          px[y * (st / 4) + x] = 0x7C5CFF;   /* SCH_ACCENT */
+    }
+  }
+
   // On a verbose boot, put the kernel log on the screen from the first line.
   //
   // Everything below talks to the serial port and nothing else until the first