libc: add bios/text.h BIOS text-output API

Direct __naked z80 wrappers over BIOS LP_PRINT_*/LP_SET_PLACE/LP_GET_PLACE
(RST 8, opcodes 081h-08Eh) for fast text rendering — a single BIOS call
replaces a wrchar() loop for constant-attribute/constant-character runs.
Covered by tests/bios_text.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/Makefile

@@ -26,6 +26,7 @@ LIBC_C := \
     libc/conio/conio.c \
     libc/conio/cprintf.c \
     libc/conio/text_palette.c \
+    libc/bios/text.c \
     libc/io/dir.c \
     libc/video/videomode_raw.c \
     libc/video/palette.c \