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>

scripts/qemu_session.py

@@ -30,6 +30,10 @@ the stick's, plan 2026-10-26-stick-1-screen-and-keyboard.md):
   usb_kbd                     the only keyboard a USB one (--make "QEMU_EXTRA='-machine
                               i8042=off -device usb-kbd,bus=xhci0.0,port=3'"): it types
   no_framebuffer              no display (-vga none): serial alone, 7  ok
+  alttab_after_logout         after logout, Alt+TAB twice: the screen stays black
+  alttab_before_pull          Alt+TAB, then the drive pulled: black; re-attach draws the console
+  emergency_pull              EMERGENCY_CONSOLE_ENABLED=1: Zuse added and pulled, the screen
+                              still shows the running console, and it types
   virtio_kbd                  aarch64/riscv64: QEMU's virtio keyboard types 7  ok
 
 Needs only Python 3. One QEMU at a time (.claude/CLAUDE.md).
@@ -49,6 +53,7 @@ ap.add_argument("--no-zuse", action="store_true")
 a = ap.parse_args()
 
 NO_ZUSE_MODES = {"headless", "hotplug", "midpull", "waitpull", "typeahead", "emergency",
+                 "alttab_after_logout", "alttab_before_pull", "emergency_pull",
                  "screen_dark_until_login", "screen_console_after_login", "screen_dark_after_logout",
                  "kbd_typeahead"}
 label = a.label or f"{a.mode}-{a.arch}"
@@ -176,6 +181,11 @@ def key_line(line):
         time.sleep(0.1)
 
 
+def alt_tab():
+    qcmd("send-key", keys=[{"type": "qcode", "data": "alt"}, {"type": "qcode", "data": "tab"}])
+    time.sleep(1)
+
+
 SHOTS = 0
 
 
@@ -348,6 +358,54 @@ elif m in ("no_framebuffer", "virtio_kbd"):
         key_line("3 4 + .")
     check("7  ok", wait_for(PROMPT, 60, n) and re.search(r"7\s+ok", text()) is not None)
 
+elif m in ("alttab_after_logout", "alttab_before_pull"):
+    add_zuse(1)
+    check("[zuse@Hera] ok> after attach", wait_for(PROMPT, 120))
+    n = count(PROMPT)
+    key_line("3 4 + .")
+    check("7  ok typed", wait_for(PROMPT, 60, n))
+    if m == "alttab_before_pull":
+        alt_tab()                                   # to the graphics view, her session on screen
+    del_zuse(1)
+    check("session ended on detach", wait_for(r"Zuse: session ended -- reattach to re-authenticate", 60))
+    time.sleep(5)
+    if m == "alttab_after_logout":
+        alt_tab()
+        alt_tab()
+        time.sleep(3)
+    _, _, lit, _ = screen()
+    check(f"the screen is black after logout ({lit} lit)", lit == 0)
+    if m == "alttab_before_pull":
+        p = count(PROMPT)
+        add_zuse(2)
+        check("[zuse@Hera] ok> again", wait_for(PROMPT, 120, p))
+        time.sleep(3)
+        _, _, lit1, px1 = screen()
+        check(f"the console is drawn again ({lit1} lit)", lit1 >= 500)
+        n = count(PROMPT)
+        key_line("1 2 + .")
+        check("3  ok typed", wait_for(PROMPT, 60, n))
+        time.sleep(3)
+        _, _, _, px2 = screen()
+        check("the screen changed with what was typed", px2 != px1)
+
+elif m == "emergency_pull":
+    check("ok> with no drive", wait_for(ANYPROMPT, 60))
+    add_zuse(1)
+    check("Zuse: identity confirmed", wait_for(r"Zuse: identity confirmed from attached thumbdrive", 120))
+    time.sleep(3)
+    del_zuse(1)
+    check("session ended on detach", wait_for(r"Zuse: session ended -- reattach to re-authenticate", 60))
+    time.sleep(5)
+    _, _, lit1, px1 = screen()
+    check(f"the screen still shows the console ({lit1} lit)", lit1 >= 500)
+    n = count(ANYPROMPT)
+    key_line("3 4 + .")
+    check("7  ok typed after the pull", wait_for(ANYPROMPT, 60, n) and re.search(r"7\s+ok", text()) is not None)
+    time.sleep(3)
+    _, _, _, px2 = screen()
+    check("the screen changed with what was typed", px2 != px1)
+
 else:
     check(f"known mode {m}", False)