Stage D: parked-idle VMs are killable, KILL unregisters; fix uninitialized pending_reap (FABRIC-3.8 §3.5, G6)

sk_vm_parked_idle(): saved context + exactly its own IDLE frame.
capsule_vm_kill_refusal() is the single kill rule, shared by
capsule_vm_kill() and the KILL word: refuse SWITCHED_OUT mid-task, parked
with mail pending, or the VM running; allow parked-idle. capsule_vm_kill()
now unregisters the switch slot (G6).

Finding fixed: vm_registry_alloc() never initialized pending_reap (kmalloc
does not zero); C-b's reap_pending walks every participant, so garbage
reaped a healthy VM (seen live before any KILL). Now zeroed at alloc.

Probe (reverted): baseline KILL refused, slots 3->4->4 (FAIL, proves G6);
with Stage D dprobe parked idle, KILL dead, slots 3->4->3, PASS on amd64/
aarch64/riscv64. Acceptance: dict_hash unchanged, relay 3, idle growth 0.

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

src/starkernel/capsule/capsule_birth.c

@@ -38,6 +38,7 @@
 #include "starkernel/session.h" /* session_register()/session_set_pinned() -- FABRIC-2.md §H.12 step 5 */
 #include "starkernel/timer.h" /* timer_calibration_record()->vm_mode -- FABRIC-2.md §I.5 CONTRIB trust tier */
 #include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 4 -- capsule_vm_force_reap() */
+#include "starkernel/vm/switch.h" /* FABRIC-3.8.md §3.5 -- sk_vm_parked_idle(), current VM */
 #include "starkernel/vm/kernel_hermes.h" /* FABRIC-3.6.md task 3.2 -- sk_hermes_channel_subscribe() */
 #include "word_source/include/framebuffer_words.h" /* FABRIC-3.6.md task 1.8 -- register_framebuffer_words() */
 #include "vm.h"
@@ -204,6 +205,14 @@ static VMRegistryEntry *vm_registry_alloc(void) {
                                           * out of scope" for making this dynamic */
     node->entry.vm_ptr             = (void *)0;
     node->entry.stadium_patron_cell = STADIUM_CELL_NONE;
+    /* FABRIC-3.8.md Stage D finding (2026-09-23): was never initialized --
+     * kmalloc does not zero, so every born VM started with whatever
+     * pending_reap the allocation held. Latent while only the old Stage 3
+     * checkpoint read it (on a timer-picked target); live once Stage C-b's
+     * sk_vm_switch_reap_pending() began walking every participant at every
+     * safe point -- a garbage non-zero reaped a healthy VM (seen live:
+     * "REAP: dprobe tombstone reclaimed" before any KILL). */
+    node->entry.pending_reap       = 0;
     for (i = 0; i < VM_NAME_MAX; i++) node->entry.name[i] = '\0';
     node->next = (void *)0;
 
@@ -409,6 +418,33 @@ static void dispatch_init_forth(void *vm_ctx) {
  * VM Kill
  *===========================================================================*/
 
+/* FABRIC-3.8.md §3.5 (Stage D, 2026-09-23). See the header. */
+const char *capsule_vm_kill_refusal(const VMRegistryEntry *entry) {
+    if (!entry || entry->state == VM_STATE_DEAD) return (const char *)0;
+
+    /* A VM parked mid-task has a live frame on its native stack that must
+     * be resumed; a VM parked at its own trampoline with nothing in
+     * progress (sk_vm_parked_idle(): exactly its own IDLE frame) has only
+     * the trampoline loop there, which nothing ever needs back. It must
+     * also have no mail waiting: those messages hold their senders' Stadium
+     * heat, and nothing would ever drain them once the VM is gone. */
+    if (entry->state == VM_STATE_SWITCHED_OUT) {
+        if (!sk_vm_parked_idle((const VM *)entry->vm_ptr)) {
+            return " refused -- switched-out mid-task (context parked)";
+        }
+        if (sk_hermes_pending_count(entry->vm_id) > 0) {
+            return " refused -- parked with mail still pending";
+        }
+    }
+
+    /* The VM physically running can't free its own stack out from under
+     * itself -- a switch participant can now be the one executing KILL. */
+    if (entry->vm_ptr && entry->vm_ptr == (void *)sk_vm_switch_current_vm()) {
+        return " refused -- it is the VM running";
+    }
+    return (const char *)0;
+}
+
 int capsule_vm_kill(const char *name) {
     VMRegistryEntry *entry;
     VM              *vm;
@@ -456,12 +492,17 @@ int capsule_vm_kill(const char *name) {
      * here would leave that saved SP pointing into freed memory, a real
      * use-after-free the moment anything ever tried to resume it. Refuse
      * rather than silently corrupt; the caller can retry once the VM has
-     * been switched back in and reaches a normal LIVE state. */
-    if (entry->state == VM_STATE_SWITCHED_OUT) {
-        console_puts("KILL: ");
-        console_puts(name);
-        console_println(" refused -- switched-out (context parked)");
-        return -1;
+     * been switched back in and reaches a normal LIVE state. Stage D
+     * narrows this to what actually needs resuming -- see
+     * capsule_vm_kill_refusal(). */
+    {
+        const char *why = capsule_vm_kill_refusal(entry);
+        if (why) {
+            console_puts("KILL: ");
+            console_puts(name);
+            console_println(why);
+            return -1;
+        }
     }
 
     vm_id = entry->vm_id;
@@ -482,6 +523,12 @@ int capsule_vm_kill(const char *name) {
         sf_free(vm);
     }
 
+    /* Stage D (§3.5): same generic cleanup capsule_vm_force_reap() already
+     * does -- a dead VM is no longer a switch participant. Before this,
+     * an ordinary KILL left its slot behind (G6), to be walked by every
+     * decision and never freed. No-op for a VM that never registered. */
+    sk_vm_switch_signal_unregister(vm_id);
+
     entry->vm_ptr = (void *)0;
     entry->state  = VM_STATE_DEAD;
     for (i = 0; i < VM_NAME_MAX; i++) entry->name[i] = '\0';