Stage 3 follow-on: message-arrival eligibility hook + trampoline-blind switch-storm fix (FABRIC-3.md §XXVIII.2)

Implements the message-arrival eligibility signal FABRIC-3.md §XXVIII.1 left
open (has_work per-slot flag, set via new SWITCH-MARK-WORK primitive from
MSG-SEND) so an idle VM never becomes a switch target purely by waiting out
the readiness threshold.

Also root-causes and fixes a second, independent switch-storm: the tick's
"who is current" check used vm_log_attributed_vm(), which can't see a VM
parked in switch.c's own raw trampoline. Replaced with a dedicated
g_switch_current_vm tracked by the switch mechanism itself, and moved
target-slot eligibility reset to the switch decision point instead of
relying on ISR polling to observe a window that can be only a few
instructions wide.

Verified live on all 3 architectures: clean boot to zuse@Hera] ok>, live
cross-VM message dispatch, and (since a quiet log looks identical to a
livelocked storm once the DoE probe is gone) confirmed genuine REPL
liveness via QMP send-key + screendump on aarch64/riscv64, not log
inspection alone.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWpNjdwPtFLuVLaAq44L9K

include/starkernel/vm/switch.h

@@ -46,6 +46,15 @@ struct VM;
 
 int sk_vm_context_switch(struct VM *from, struct VM *to);
 
+/* 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
+ * exists instead of reusing vm_log_attributed_vm(). sk_vm_switch_set_
+ * current() seeds the initial value (whoever is running before any
+ * switch has ever happened) -- call once, at Stage 3 registration time. */
+struct VM *sk_vm_switch_current_vm(void);
+void sk_vm_switch_set_current(struct VM *vm);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_SWITCH_H */