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

src/starkernel/vm/vm_core.c

@@ -920,14 +920,16 @@ void execute_colon_word(VM* vm)
          * `vm`, which could be an arbitrary number of ticks later --
          * calling it after would attribute switch_count_cumulative/
          * switch_ticks_since to the wrong (resume) event instead of the
-         * actual switch-out this counts. */
+         * actual switch-out this counts. It also resets target_id's own
+         * eligibility state directly (2026-09-14 correction) -- see its
+         * own doc comment for why that can't be left to ISR polling. */
         if (sk_vm_at_outermost_interpret()) {
             VMUuid target_id = sk_vm_switch_signal_take_pending();
             if (!vm_uuid_equal(target_id, vm_uuid_none())) {
                 VMRegistryEntry target_entry;
                 if (capsule_vm_registry_get(target_id, &target_entry) == 0 &&
                     target_entry.vm_ptr && target_entry.vm_ptr != (void *)vm) {
-                    sk_vm_switch_signal_note_switch_performed();
+                    sk_vm_switch_signal_note_switch_performed(target_id);
                     sk_vm_context_switch(vm, (VM *)target_entry.vm_ptr);
                     /* Resumes here once something later switches back. */
                 }