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/fdt.h

@@ -167,4 +167,37 @@ const void* fdt_next_child_node(const void* fdt, const void* parent,
 const void* fdt_find_prop_in_node(const void* fdt, const void* node,
                                    const char* name, uint32_t* len_out);
 
+/**
+ * @brief Is the structure block whole?
+ * @c fdt_valid() checks the header; this checks also that the structure
+ * block ends with its end token, as a blob that has not been cut short
+ * does.  A reader that takes a property's absence to mean its default
+ * (a UART's "reg-shift") must not be handed half a tree.
+ * @return 1 if @p fdt is valid and its structure block ends with FDT_END.
+ */
+int fdt_complete(const void* fdt);
+
+/**
+ * @brief The root node.
+ * @param fdt  Blob; checked with @c fdt_valid().
+ * @return Handle to the root node, for @c fdt_find_child() and
+ *         @c fdt_find_prop_in_node(), or NULL.
+ */
+const void* fdt_root_node(const void* fdt);
+
+/**
+ * @brief Find a direct child of a node by its whole name.
+ * The name is matched exactly, unit address included ("serial@10000000"
+ * is not "serial"), and is given with its length so that one component of
+ * a path can be passed without copying it.  Only @p parent's own children
+ * are looked at, never their descendants.
+ * @param fdt       Blob; checked with @c fdt_valid().
+ * @param parent    Node handle; NULL is safe and returns NULL.
+ * @param name      The child's name; need not be NUL-terminated.
+ * @param name_len  Its length in bytes.
+ * @return Handle to the child, or NULL if @p parent has none of that name.
+ */
+const void* fdt_find_child(const void* fdt, const void* parent,
+                            const char* name, uint32_t name_len);
+
 #endif /* STARKERNEL_FDT_H */