fix(hermes): one delivery path -- a VM takes its own messages on its own turn, never on another's stack

FABRIC-3.5 SXXXII, closing FABRIC-3 SXXIX bug 2 (SXXXVI).  A send marks the
receiver for the switch signal; the VM's own loop on its native stack takes
its messages and yields; Hera's idle beat makes the pending switch instead
of running other VMs' messages on her stack; every VM gets switcher turns
from birth; the drain refuses any call outside the VM's own context (the
test: it held every relayed line before, none now).

Found on the way: amd64's first-entry frame left rsp 8 bytes off the SysV
entry alignment (#GP on the first interpreter run on a VM's own stack), and
a VM yielding from its loop was marked SWITCHED_OUT, which stopped console
relay after one message and would have made KILL refuse it.

Relay of four lines to a WIREBIND identity on all three ISAs, five
hot-plugged identities, and standard acceptance (POST 1052, hash
0x08873e0f44b7cb2a) all pass.

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

kernel/include/starkernel/vm/switch.h

@@ -46,6 +46,13 @@ struct VM;
 
 int sk_vm_context_switch(struct VM *from, struct VM *to);
 
+/* Performs the switch the switch signal has pending, if any, from `from`
+ * -- the VM that owns the native stack now running. Returns 1 once it has
+ * switched and been switched back to, 0 when there was nothing to do.
+ * Called at the colon-word checkpoint (vm_core.c) and at Hera's idle beat
+ * (repl.c). */
+int sk_vm_switch_service_pending(struct VM *from);
+
 /* FABRIC-3.md §XXVIII Stage 3 follow-on (2026-09-14): which VM is
  * currently physically running, tracked by the switch mechanism itself --
  * see switch.c's own doc comment on g_switch_current_vm for why this
@@ -55,6 +62,10 @@ int sk_vm_context_switch(struct VM *from, struct VM *to);
 struct VM *sk_vm_switch_current_vm(void);
 void sk_vm_switch_set_current(struct VM *vm);
 
+/* True when at most one vm_interpret() is in progress, i.e. the word now
+ * running belongs to the outermost text on this stack (vm_core.c). */
+int sk_vm_at_outermost_interpret(void);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_SWITCH_H */