libc: add Sprinter palette API (graphics + text planes)

palette.h/video/palette.c wrap BIOS $A4 PIC_SET_PAL/PIC_GET_PAL and $A6
SET_PAL_INIT for the 8 palette pages (0-3 graphics, 4-7 text planes).
gfx_palette.c and conio/text_palette.c are thin per-domain wrappers;
text_palette.c maps the text "plane" 0..3 (paper/ink/blink-paper/
blink-ink) onto BIOS pages 4..7. Covered by tests/text_palette.

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

tests/text_palette/Makefile

@@ -0,0 +1,5 @@
+# Build text_palette.exe — uses lib/sprinter.lib in TINY memory mode.
+
+PROJ_ROOT := $(abspath $(CURDIR)/../..)
+EXAMPLE   := text_palette
+include $(PROJ_ROOT)/app.mk