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>

examples/banked/banked.c

@@ -13,7 +13,7 @@ extern uint8_t bank_pages[];   /* filled by crt0_banked.s */
 static void show_w3(const char *who)
 {
     puts(who);
-    print_hex(_io_page_w3);     /* current physical page mapped in window 3 */
+    hex8(_io_page_w3);     /* current physical page mapped in window 3 */
     putchar('\n');
 }
 
@@ -26,10 +26,10 @@ int main(void)
     show_w3("HOME: window 3 phys page = ");
 
     puts("HOME: bank_pages[1] (BANK1 phys) = ");
-    print_hex(bank_pages[1]);
+    hex8(bank_pages[1]);
     putchar('\n');
     puts("HOME: bank_pages[2] (BANK2 phys) = ");
-    print_hex(bank_pages[2]);
+    hex8(bank_pages[2]);
     putchar('\n');
 
     puts("HOME: calling bank1_func(42)...");