feat(v4): the screen and the keyboards

The stick, increment 1, task 2 (plan 2026-10-26-stick-1-screen-and-keyboard;
docs/v4.0.0/STICK.md 2, 5.3, 5.4). On the v4 path:
- the framebuffer is brought up after POST, cleared and dark: console text
  goes to serial only ("splash only, log on serial") until Zuse logs in;
  then the screen is the console (TTF, a blinking cursor, as v3's), and
  when she logs out it is cleared and dark again (console_fb_mirror(),
  console_fb_clear(); the mirror is on by default, so v3 is unchanged);
- the line editor and the node's KEY read every keyboard -- PS/2, USB, the
  virtio keyboard -- through the shared bridge (sk_input_getc()); the
  headless wait drops keyboard input as well as serial bytes;
- an EMERGENCY_CONSOLE_ENABLED build has the screen from the start.
The framebuffer setup is one helper both paths call (kernel_main.c).

Red (before): screen_dark_until_login 153051 (10835 lit), console/keys
153309, logout 153609, kbd_typeahead 153918, virtio_kbd aarch64 154245
and riscv64 154718; no_framebuffer 155546 passed before and after.
Green: 155926 (0 lit), 160156, 160409 (0 lit after logout), 160630,
virtio_kbd 161414 and 161820, no_framebuffer 161206; usb_kbd with the USB
keyboard the only one (-machine i8042=off) 164548, v3 control 164757;
the identity gate's sessions 162245-163802 all pass; v3 typed by keyboard
164009, 164115. (usb_kbd 160857 and 164313 failed in the harness:
input-send-event's device names a display, not a keyboard.)
Accepted: suite EXIT 0; three clean qemu boots, POST 538/538, dict_hash
0xfec8c47cfc6f437f unchanged (174704, 174911, 175315); test-32, 39
modules, 0 failures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kernel/include/starkernel/console.h

@@ -96,6 +96,15 @@ void console_fb_init(const FramebufferInfo *info, FbPixelFormat fmt);
  */
 void console_fb_enable_ttf(void);
 
+/* Whether console text is drawn on the framebuffer too (on by default, as
+ * v3's console has it). The v4 boot turns it off until Zuse logs in, and
+ * off again when she logs out: "splash only, log on serial"
+ * (docs/v4.0.0/STICK.md 2). The serial line always has everything. */
+void console_fb_mirror(int on);
+
+/* Erase the screen and put the cursor home (when the framebuffer is up). */
+void console_fb_clear(void);
+
 /**
  * FABRIC-0.md item 4.4q: thin wrappers over vt100_scroll_back()/
  * vt100_scroll_fwd() -- see those functions' doc comments for the full