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/console.h

@@ -102,7 +102,9 @@ void console_fb_enable_ttf(void);
  * (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). */
+/* Erase the screen and forget everything the terminal has shown -- the
+ * on-screen text and the scrollback, so nothing can be drawn back -- and
+ * leave the terminal visible, cursor home (when the framebuffer is up). */
 void console_fb_clear(void);
 
 /**

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

kernel/src/hal/console.c

@@ -262,8 +262,9 @@ void console_fb_mirror(int on) { g_fb_mirror = on ? 1 : 0; }
 
 void console_fb_clear(void)
 {
-    if (fb_is_available()) vt100_puts("\x1b[2J\x1b[H");
+    if (fb_is_available()) vt100_reset();
 }
+
 static uint64_t g_console_tx_count; /* monotonic console_putc() counter (BSS) */
 
 void console_set_vm_name(const char *name) {
@@ -487,7 +488,9 @@ void console_fb_scroll_fwd(uint32_t n)
 
 void console_fb_toggle_graphics(void)
 {
-    if (fb_is_available()) {
+    /* Not while the screen is off (the v4 boot, headless): Alt+TAB would
+     * draw the terminal back (fixed 2026-10-26). v3's is always on. */
+    if (fb_mirrored()) {
         vt100_toggle_graphics();
     }
 }

kernel/src/hal/vt100.c

@@ -631,6 +631,39 @@ static void scrollback_redraw(void)
  * reuse scrollback_redraw() to repaint the terminal's current on-screen
  * content from that untouched logical state -- restored exactly as it
  * logically stood, not merely as it last looked on screen. */
+/* Everything the terminal has shown, forgotten: the screen filled with the
+ * background, the terminal visible again (out of the graphics view), and
+ * the on-screen text, the scrollback and the boot-mode snapshot of both
+ * emptied, so that nothing can be drawn back -- by Alt+TAB's redraw or by
+ * scrolling. The v4 boot's logout (docs/v4.0.0/STICK.md 2: nothing of a
+ * session is left on the screen; fixed 2026-10-26, the stick's increment 1
+ * final review: ESC[2J cleared the pixels only, and Alt+TAB drew her
+ * session back from g_shadow). */
+void vt100_reset(void)
+{
+    uint32_t r, c;
+
+    if (!g_vt.initialized) return;
+    g_terminal_visible = 1;
+    if (g_shadow) {
+        for (r = 0; r < g_vt.rows; r++) {
+            char *row = g_shadow + (size_t)r * g_line_stride;
+            for (c = 0; c < g_vt.cols; c++) row[c] = ' ';
+            row[g_vt.cols] = '\0';
+        }
+    }
+    g_ring_count      = 0;
+    g_ring_head       = 0;
+    g_scroll_offset   = 0;
+    g_boot_ring_count = 0;
+    g_boot_rows       = 0;
+    g_vt.cx = g_vt.cy = 0;
+    g_vt.saved_cx = g_vt.saved_cy = 0;
+    reset_attrs();
+    reset_parser();
+    fb_fill_rect(0, 0, fb_width(), fb_height(), g_vt.def_bg);
+}
+
 void vt100_toggle_graphics(void)
 {
     if (!g_ttf_ready || g_glyph_mode != VT_GLYPH_TTF) return;

kernel/src/v4/sk_v4_identity.c

@@ -140,9 +140,13 @@ static void sk_v4_drive_left(blkio_dev_t *dev)
     if (dev != g_zuse_dev) return;
     g_zuse_session = 0;
     g_zuse_dev = (struct blkio_dev *)0;
-    /* nothing of her session is left on the screen; it is dark again */
+    /* nothing of her session is left on the screen; it is dark again --
+     * except in a build with EMERGENCY_CONSOLE_ENABLED, whose console needs
+     * no login and goes on running (the stick's increment 1 final review) */
+#if !EMERGENCY_CONSOLE_ENABLED
     console_fb_clear();
     console_fb_mirror(0);
+#endif
     console_println("Zuse: session ended -- reattach to re-authenticate");
 }