Drop print_hex.c — replaced by hex8() from hex_print.c

print_hex(uint8_t) was an early bare-metal helper doing exactly what
hex8() in the freshly-ported hex_print.c does (two-digit hex via
putchar()).  hex8() is smaller (asm cp/sbc/daa nibble trick, no LUT)
and consistent with the dec8/hex16/dec16/hex32/dec32 family.

  • Replaced print_hex() calls with hex8() in examples/banked and
    examples/bankedbg.
  • Removed libc/stdio/print_hex.c, dropped its prototype from
    sprinter.h and its entry from lib/Makefile.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

libc/stdio/print_hex.c

@@ -1,15 +0,0 @@
-/*
- * print_hex — print a single byte as two uppercase hex digits.
- *
- * No printf yet; this is what bare-metal debug looks like in stage 3.
- */
-
-#include <stdio.h>
-#include <sprinter.h>
-
-void print_hex(uint8_t v)
-{
-    static const char digits[] = "0123456789ABCDEF";
-    putchar(digits[(v >> 4) & 0x0F]);
-    putchar(digits[v & 0x0F]);
-}