Stage 3 follow-on: fix stack-ownership corruption + DoE switch columns (FABRIC-3.md §XXVIII.1)

DoE CSV gained 6 switch-signal columns (switch_count_cumulative,
switch_current_slot, switch_*_readiness, switch_ticks_since), and verifying
them with a boot-time HB-ON probe surfaced a real livelock: the preemption
checkpoint could fire inside a VM-EXEC-nested execute_colon_word() call and
switch away from a stack it didn't own, parking a borrowed region of the
caller's stack under the wrong VM's saved-context pointer. The trampoline
bounce was the visible (safe) half of this; the corruption was the quiet
half, live in every prior "clean" Stage 3 boot without ever showing up in
the log.

Fixed by gating the checkpoint on being at the outermost vm_interpret()
call (g_vm_interpret_depth / sk_vm_at_outermost_interpret(), vm_core.c),
per Bob's decision. Also fixed two related bugs found in the same pass:
g_switch_back_to was a single global stale after first entry, now per-VM
state (native_switch_back_to); note_switch_performed() fired on resume
instead of switch-out, now called before the switch.

Verified on all 3 architectures: steady log growth (no freeze), zero
leaked QEMU processes, DoE columns internally consistent, Hermes/Artemis
confirmed genuinely executing (not just trampoline-bouncing). Temporary
HB-ON boot probe reverted after capture.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016UNhH1mhi52i6Qihh7ZV5S

src/starkernel/capsule/capsule_vm_switch_signal.c

@@ -56,6 +56,11 @@ static int    g_slot_count;
 static int    g_pending;
 static VMUuid g_pending_target;
 
+/* DoE CSV exposure (FABRIC-3.md §XXVIII Stage 3 follow-on, 2026-09-13). */
+static int      g_current_slot_cached = -1;
+static uint64_t g_switch_count;
+static uint32_t g_ticks_since_switch;
+
 static int slot_for_vm_id(VMUuid vm_id) {
     int i;
     for (i = 0; i < g_slot_count; i++) {
@@ -81,6 +86,9 @@ void sk_vm_switch_signal_tick(void) {
     current_slot = slot_for_vm_id(current->stadium_vm_id);
     if (current_slot < 0) return; /* current VM isn't a registered participant */
 
+    g_current_slot_cached = current_slot;
+    g_ticks_since_switch++;
+
     for (i = 0; i < g_slot_count; i++) {
         if (i == current_slot) {
             g_slots[i].readiness = 0;
@@ -106,4 +114,36 @@ VMUuid sk_vm_switch_signal_take_pending(void) {
     return g_pending_target;
 }
 
+void sk_vm_switch_signal_note_switch_performed(void) {
+    g_switch_count++;
+    g_ticks_since_switch = 0;
+}
+
+uint64_t sk_vm_switch_signal_switch_count(void) {
+    return g_switch_count;
+}
+
+uint32_t sk_vm_switch_signal_ticks_since_switch(void) {
+    return g_ticks_since_switch;
+}
+
+int sk_vm_switch_signal_current_slot(void) {
+    return g_current_slot_cached;
+}
+
+int sk_vm_switch_signal_slot_count(void) {
+    return g_slot_count;
+}
+
+uint32_t sk_vm_switch_signal_readiness(int slot) {
+    if (slot < 0 || slot >= g_slot_count) return 0;
+    return g_slots[slot].readiness;
+}
+
+uint32_t sk_vm_switch_signal_readiness_of(VMUuid vm_id) {
+    int slot = slot_for_vm_id(vm_id);
+    if (slot < 0) return 0;
+    return g_slots[slot].readiness;
+}
+
 #endif /* __STARKERNEL__ */