fix(v4): the identity gate's final review, items 1-4

From the fresh whole-branch review of the identity gate (With fixes; no
Critical, four Important):
1. A drive pulled while a line ran: the drives are looked at before every
   prompt (sk_v4_identity_check_now), so no prompt is shown once she is
   gone and what was typed after the pull does not run as Zuse.
2. Pulled while a line WAITS: the wait is ended at logout, by an empty
   line, and not carried into her next session (Captain Bob, 2026-10-26:
   "end the waiting line at logout"); re-attaching brings the prompt back.
3. EMERGENCY_CONSOLE_ENABLED: v4 honours it as v3 does -- a bring-up build
   has a prompt with no drive (boards/BRINGUP.md A2).
4. Keys typed at a headless machine are read and dropped, not run as
   whoever logs in next. (v3 has the same gap, sk_repl_headless_wait;
   reported, not changed.)

Each a QEMU session red then green (amd64): midpull 111650 -> 112953,
waitpull 111907 -> 113154, emergency 112524 -> 113609, typeahead
112248 -> 113359; headless 113800 and hotplug 114043 still pass.
Accepted: three clean qemu boots to [zuse@Hera] ok>, POST 538/538,
dict_hash 0xfec8c47cfc6f437f on all three; hotplug on aarch64 and
riscv64; make -C v4 test test-32 sanitize hosted-check EXIT 0;
make -C v4 test-32 0 failures.

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

kernel/include/starkernel/v4/sk_v4_identity.h

@@ -20,8 +20,13 @@ int sk_v4_identity_present(void);
  * with nothing typed. */
 void sk_v4_identity_beat(void);
 
-/* Headless: no prompt, no input, until Zuse's session begins (v3's
+/* The USB drives that have arrived or left, now, not waiting for the next
- * sk_repl_headless_wait()). */
+ * beat: before each prompt, so that a drive pulled while the last line ran
+ * is seen before a prompt is shown. */
+void sk_v4_identity_check_now(void);
+
+/* Headless: no prompt, and what is typed is read and dropped, until Zuse's
+ * session begins (v3's sk_repl_headless_wait()). */
 void sk_v4_identity_wait(void);
 
 #endif /* STARKERNEL_V4_SK_V4_IDENTITY_H */

kernel/src/v4/sk_v4.c

@@ -77,8 +77,10 @@ static void sk_v4_serve(v4_node *node, unsigned request)
 
 /* Read a line from the keyboard: characters are echoed, backspace rubs one
  * out, Enter ends it.  Returns its length; or -1 when, with nothing typed,
- * Zuse's session has ended (her drive was pulled): the caller goes headless.
+ * Zuse's session has ended (her drive was pulled): the caller goes headless
- * While nothing is typed the idle beat runs, as v3's sk_console_readline(). */
+ * (not in a build with EMERGENCY_CONSOLE_ENABLED, whose console needs no
+ * login: v3's sk_console_readline() the same).  While nothing is typed the
+ * idle beat runs, as v3's. */
 static int sk_v4_readline(void)
 {
     int len = 0;
@@ -87,7 +89,9 @@ static int sk_v4_readline(void)
         if (c < 0) {
             if (len == 0) {
                 sk_v4_identity_beat();
+#if !EMERGENCY_CONSOLE_ENABLED
                 if (!sk_v4_identity_present()) return -1;
+#endif
             } else {
                 heartbeat_service();
             }
@@ -134,9 +138,18 @@ void sk_v4_run(void (*chain)(void))
     for (;;) {
         int len;
         int how;
+        int logged_out = 0;
 
-        /* no identity, no console: headless until Zuse's drive is checked */
+#if !EMERGENCY_CONSOLE_ENABLED
+        /* no identity, no console: headless until Zuse's drive is checked.
+         * The drives are looked at first, now: one pulled while the last
+         * line ran is seen before a prompt is shown (the identity gate's
+         * final review, item 1).  Not in a build with
+         * EMERGENCY_CONSOLE_ENABLED, whose console needs no login (v3's
+         * kernel_main.c and repl.c the same; item 3). */
+        sk_v4_identity_check_now();
         if (!sk_v4_identity_present()) sk_v4_identity_wait();
+#endif
         console_puts(SK_V4_PROMPT);
         len = sk_v4_readline();
         if (len < 0) continue;
@@ -146,13 +159,26 @@ void sk_v4_run(void (*chain)(void))
          * error, and then the line just typed is done (v4/boot.h). */
         while (how == V4_BOOT_LINE_WAITING) {
             len = sk_v4_readline();
-            if (len < 0) { sk_v4_identity_wait(); continue; }
+            if (len < 0) {
+                /* Zuse's session ended while the line waited: its wait is
+                 * ended now, by an empty line, and not carried into the
+                 * next session (Captain Bob, 2026-10-26: "end the waiting
+                 * line at logout").  How it ended is shown; the empty line
+                 * is then done, and shows nothing. */
+                how = v4_boot_line(&boot, "", 0u);
+                if (how == V4_TEXT_ERROR) console_puts(" ERROR\n");
+                else if (how == V4_TEXT_COMPLETED) console_puts(" ok\n");
+                if (how == V4_TEXT_ERROR || how == V4_TEXT_COMPLETED) how = v4_boot_line(&boot, 0, 0);
+                logged_out = 1;
+                break;
+            }
             how = v4_boot_line(&boot, sk_v4_text, (unsigned)len);
             if (how == V4_TEXT_ERROR) console_puts(" ERROR\n" SK_V4_PROMPT);
             else if (how == V4_TEXT_COMPLETED) console_puts(" ok\n" SK_V4_PROMPT);
             else break;
             how = v4_boot_line(&boot, 0, 0);
         }
+        if (logged_out && (how == V4_TEXT_ERROR || how == V4_TEXT_COMPLETED)) continue;
         if (how == V4_TEXT_COMPLETED) console_puts(" ok\n");
         else if (how == V4_TEXT_ERROR) console_puts(" ERROR\n");
         else if (how == V4_BOOT_LINE_REFUSED) console_puts("Message refused\n ERROR\n");      /* the node's wire is full: error 19's words */

kernel/src/v4/sk_v4_identity.c

@@ -191,10 +191,26 @@ void sk_v4_identity_beat(void)
     }
 }
 
+void sk_v4_identity_check_now(void)
+{
+    g_last_beat = heartbeat_ticks();
+    sk_v4_idle();
+}
+
+/* What was typed at a headless machine is no one's: it is read and dropped,
+ * so that it does not run as whoever logs in next (the identity gate's
+ * final review, item 4, 2026-10-26). */
+static void sk_v4_discard_input(void)
+{
+    while (console_getc() >= 0) { }
+}
+
 void sk_v4_identity_wait(void)
 {
     while (!g_zuse_session) {
+        sk_v4_discard_input();
         sk_v4_identity_beat();
         arch_relax();
     }
+    sk_v4_discard_input();
 }