tests: add hello2, simple, banktest smoke tests

hello2 — stdio vs conio output API comparison (fast/no-attr vs
slower/attributed), also exercises textcolor/textbackground.
simple — packed 2-bit style array bit-twiddling smoke test.
banktest — __banked function calls across two explicit banks in huge
memory mode.

hello2.c updated to call the renamed gettextmode/settextmode (see the
conio rename in 5e5e70d).

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

tests/hello2/hello2.c

@@ -0,0 +1,82 @@
+/*
+ * hello — smoke test for the two output APIs:
+ *
+ *   stdio (printf/puts/putchar)  — FAST, no attribute (ambient colour)
+ *   conio (cprintf/cputs/putch)  — SLOWER, applies textcolor/textbackground
+ *
+ * Demonstrates textcolor/textbackground (Turbo-C-compatible names).
+ */
+
+#include <stdio.h>
+#include <conio.h>
+#include <gfx.h>
+#include <sprinter.h>
+#include <sprinter_exit.h>
+#include <errno.h>
+
+int main(void)
+{
+    errno = -1;
+    int16_t a0 = 0, a1 = 0;
+    a0 = get_text_attr();
+
+    uint8_t vMode = gettextmode();
+    settextmode(TEXT_MODE_80x32);
+
+    /* Wipe the screen with a known attribute so stdio printf/puts inherit
+     * a clean colour for the cursor. */
+    clrscr_attr(COLOR(COLOR_LIGHTGRAY, COLOR_BLACK));
+
+    /* --- stdio set: fast, no attribute ------------------------------- */
+    puts("hello (stdio) - fast PCHARS, ambient colour");
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    /* --- conio set: applies textcolor / textbackground --------------- */
+    textcolor(COLOR_YELLOW);
+    textbackground(COLOR_BLUE);
+    cputs("conio: yellow on blue\r\n");
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    textcolor(COLOR_LIGHTRED);
+    textbackground(COLOR_BLACK);
+    cprintf("cprintf %d %d %s\r\n", 42, -7, "args");
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    /* set_text_attr / textattr replace the whole byte. */
+    textattr(COLOR(COLOR_LIGHTGREEN, COLOR_BLACK));
+    cputs("conio: light-green via textattr\r\n");
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    /* Opting out of attribute control — conio falls back to fast path. */
+    set_text_attr(KEEP_EXIST_ATTR);
+    cputs("conio with KEEP_EXIST_ATTR (fast path)\r\n");
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    a1 = get_text_attr();
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    /* Back to a normal attribute so the goodbye reads cleanly. */
+    textattr(COLOR(COLOR_LIGHTCYAN, COLOR_BLACK));
+
+    printf("printf: a0=%d (initial attr)\n", a0);
+
+    cprintf("a0=%d a1=%d now=%d errno=%d\n\r",
+           a0, a1, get_text_attr(), errno);
+    // cprintf("a0=%d a1=%d",
+    //        a0, a1);
+    // cprintf(" now=%d errno=%d\n\r",
+    //        get_text_attr(), errno);
+#ifdef DEBUG_RT
+    printf("w2_self_allocated = %u\n", w2_self_allocated);
+#endif
+    cputs("Press any key to exit...");
+
+    (void)getchar();
+    settextmode(vMode);
+    return 0;
+}