fix(v4): nothing of a session left on screen after logout

From the fresh review of the stick's increment 1 (With fixes):
- Critical: after logout Zuse's last screen, her prompt included, could be
  drawn back on a headless machine. The logout clear (ESC[2J) blanked the
  pixels only: Alt+TAB, still read while headless, redrew the terminal
  from its saved text; and if she had Alt+TABbed to the graphics view
  before the pull, the clear did nothing and her session stayed on screen
  (and the console did not draw at her next login). Now vt100_reset()
  forgets everything the terminal has shown -- the on-screen text, the
  scrollback and their boot snapshots -- makes it visible again and fills
  the screen; and Alt+TAB does nothing while the screen is off (v3's never
  is, so v3 is unchanged).
- Important: an EMERGENCY_CONSOLE_ENABLED build no longer clears the screen
  when Zuse's drive is pulled: its console goes on running.

Red: alttab_after_logout 183308 (4515 lit), alttab_before_pull 183529
(4539 lit; the console not redrawn), emergency_pull 183759 (0 lit).
Green: 184115, 184328, 184552; regression 184805-185956 (screens,
keyboard, hotplug, emergency, v3 typed by keyboard) all pass.
Accepted: 190111, 190312, 190710, POST 538/538, dict_hash
0xfec8c47cfc6f437f; test-32, 39 modules, 0 failures.

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

kernel/include/starkernel/vt100.h

@@ -110,6 +110,11 @@ void vt100_puts(const char *s);
  */
 void vt100_toggle_graphics(void);
 
+/* Forget everything the terminal has shown: the screen filled with the
+ * background, the terminal visible, the on-screen text, the scrollback and
+ * the boot-mode snapshot emptied, the cursor home. */
+void vt100_reset(void);
+
 /**
  * Draw a solid block cursor at the terminal's current position. Static,
  * not blinking. Idempotent -- safe to call after every keystroke, since