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>

libc/conio/text_palette.c

@@ -0,0 +1,42 @@
+/*
+ * text_palette.c — text-mode palette wrappers.
+ *
+ * Thin layer over libc/video/palette.c. Translates the text "plane
+ * number" 0..3 (paper/ink/blink-paper/blink-ink) into the underlying
+ * BIOS palette page 4..7 used by $A4.
+ *
+ * For an introduction to the four-plane text colour model see
+ * <palette.h> (top-of-file doc-block).
+ */
+
+#include <stdint.h>
+#include <conio.h>
+#include <palette.h>
+
+void text_pal_load(uint8_t plane, uint8_t start, uint8_t count,
+                   const uint8_t *data)
+{
+    pal_load((uint8_t)(plane + 4u), start, count, data);
+}
+
+void text_pal_set_color(uint8_t plane, uint8_t attr,
+                        uint8_t r, uint8_t g, uint8_t b)
+{
+    pal_set_color((uint8_t)(plane + 4u), attr, r, g, b);
+}
+
+void text_pal_get(uint8_t plane, uint8_t start, uint8_t count, uint8_t *data)
+{
+    pal_get((uint8_t)(plane + 4u), start, count, data);
+}
+
+void text_pal_get_color(uint8_t plane, uint8_t attr,
+                        uint8_t *r, uint8_t *g, uint8_t *b)
+{
+    pal_get_color((uint8_t)(plane + 4u), attr, r, g, b);
+}
+
+void text_pal_reset(void)
+{
+    pal_reset(PAL_CGA);
+}