fix(console): the serial line is found, not assumed -- from the devicetree on a board, COM1 by probe on a PC

Each board has one serial line that is its console, the lowest common
denominator beside the screen and keyboard.  Its address was QEMU's,
fixed: on a Raspberry Pi 5 the aarch64 one is ordinary RAM, written to
with every character printed; on a Milk-V Mars the riscv64 one is the
board's UART with registers four bytes apart, and the UEFI loader would
have waited for ever on its first character; on a PC with no COM1 the
prompt read 0xFF for ever.

console_attach_serial(), called before anything prints: COM1 if a UART
answers there; elsewhere the UART that /chosen/stdout-path names
(fdt_uart.c: alias, path, the address carried up through each bus's
ranges, reg-shift and reg-io-width); with no devicetree, QEMU's.  A
devicetree that names no usable console gives no line, not a guess.  A
line that does not come ready is given up, not waited on.  The riscv64
loader's early serial does the same.

tools/fdt_uart_test.c, run by hand, 19 checks plain and under the
sanitizers: devicetrees shaped like the Mars's, the Pi 5's and QEMU's,
what must find nothing, and a block cut short at every length; and the
Pi 5's real devicetree gives a PL011 at 0x107d001000.

Acceptance, one QEMU at a time: v3 on amd64 (logs/20261013-204436),
aarch64 (-204803) and riscv64 (-205001) to the zuse prompt, no unknown
word, hash 0x08873e0f44b7cb2a on all three as before.  -204600 is an
aarch64 run during which the disk images were restored under it by a
script that had been refused a second QEMU; it passed and is kept, and
was run again.  v4 on the three ISAs (-211149, -211444, -211857): POST
538 of 538, dict_hash 0xe5a57f3eb72ff94a, the typed session unchanged.
The Mars image through OpenSBI and U-Boot in QEMU took its line and its
interrupt controller from the devicetree and passed POST.

Found on the way and not mended (boards/BRINGUP.md 1.6): built with
EMERGENCY_CONSOLE_ENABLED=1 the prompt floods with empty lines.  -210442
shows it; the commit before this does the same.

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

kernel/include/starkernel/console.h

@@ -57,12 +57,25 @@
 #include <stddef.h>
 #include "uefi.h"
 
+/**
+ * Find the board's serial console line.  Must be called before anything is
+ * printed: until it is, and on a board with no such line, the console writes
+ * to no serial port and reads from none.  On a PC it is COM1, if a UART
+ * answers there; elsewhere it is the UART the devicetree names as the
+ * console (/chosen/stdout-path); with no devicetree at all it is the UART of
+ * QEMU's virt machine.  @p dtb may be NULL.
+ */
+void console_attach_serial(const void *dtb);
+
 /**
  * Initialize serial console (UART).
- * Must be called once during early kernel boot.
+ * Must be called once during early kernel boot, after console_attach_serial().
  */
 void console_init(void);
 
+/** Print one line saying which serial line the console found, or that there is none. */
+void console_describe_serial(void);
+
 /**
  * Initialize the framebuffer VT100 terminal.
  * Call after UEFI boot services have been exited and the GOP framebuffer