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/capsule/mama_forth_words.c

@@ -55,6 +55,7 @@
 #include "starkernel/capsule_vm_physics.h"
 #include "starkernel/vm/vm_internal.h"
 #include "starkernel/vm/stadium.h" /* item 4.2 -- STADIUM-* words */
+#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 follow-on -- SWITCH-MARK-WORK */
 #include "starkernel/vm/stadium_words.h" /* item 4.2 -- stadium_words_resident_heat() */
 #include "starkernel/repl.h"
 #include "starkernel/capsule_generated.h"
@@ -785,6 +786,43 @@ static void mama_word_vm_exec(VM *vm)
     /* Stack clean on exit */
 }
 
+/**
+ * @brief SWITCH-MARK-WORK ( name-caddr name-u -- )
+ * FABRIC-3.md §XXVIII Stage 3 follow-on (2026-09-14): message-arrival
+ * eligibility hook. Called from MSG-SEND (capsules/common/messaging.4th)
+ * every time a message is sent to a VM, so the Stage 3 switch signal can
+ * tell "has recently been sent something" apart from "has just been idle
+ * long enough" before ever selecting it as a switch target. Silently
+ * ignores a malformed or unknown name -- this fires on the hot path of
+ * every message send, not a user-facing command, so it must never error
+ * or spam the console.
+ */
+static void mama_word_switch_mark_work(VM *vm)
+{
+    char        name[VM_NAME_MAX];
+    cell_t      u, caddr;
+    uint32_t    i;
+    VMRegistryEntry entry;
+
+    if (vm->dsp < 2) { vm->error = 1; return; }
+
+    u     = vm_pop(vm);
+    caddr = vm_pop(vm);
+
+    if (u <= 0 || (uint32_t)u >= VM_NAME_MAX) return;
+
+    {
+        const uint8_t *p = vm_ptr(vm, (vaddr_t)caddr);
+        if (!p) return;
+        for (i = 0; i < (uint32_t)u; i++) name[i] = (char)p[i];
+        name[u] = '\0';
+    }
+
+    if (capsule_vm_find_by_name_nocase(name, &entry) == 0) {
+        sk_vm_switch_signal_mark_work(entry.vm_id);
+    }
+}
+
 /**
  * @brief VM-CALL ( cmd-caddr cmd-u vm-name-caddr vm-name-u -- n )
  * Like VM-EXEC but pops one cell from the target VM's TOS and pushes
@@ -1653,6 +1691,7 @@ void register_mama_forth_words(VM *vm)
     register_word(vm, "VM-PHYSICS-STATUS", mama_word_vm_physics_status);
     register_word(vm, "VM-STEP", mama_word_vm_step);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
+    register_word(vm, "SWITCH-MARK-WORK", mama_word_switch_mark_work);
     register_word(vm, "VM-CALL", mama_word_vm_call);
     register_word(vm, "VM-HEAT", mama_word_vm_heat);
     /* FABRIC-3.md SXX: the 8 STADIUM-* primitives register_child_vm_words()
@@ -1711,6 +1750,7 @@ void register_mama_forth_words(VM *vm)
     register_word(vm, "VM-PHYSICS-STATUS", mama_word_vm_physics_status);
     register_word(vm, "VM-STEP", mama_word_vm_step);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
+    register_word(vm, "SWITCH-MARK-WORK", mama_word_switch_mark_work);
     register_word(vm, "VM-CALL", mama_word_vm_call);
     register_word(vm, "VM-HEAT", mama_word_vm_heat);
     /* FABRIC-3.md SXX: the 8 STADIUM-* primitives register_child_vm_words()
@@ -1946,6 +1986,7 @@ void register_child_vm_words(VM *vm)
     register_word(vm, "STOP",    mama_word_stop);
     register_word(vm, "EXEC",    mama_word_exec);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
+    register_word(vm, "SWITCH-MARK-WORK", mama_word_switch_mark_work);
     register_word(vm, "VM-CALL", mama_word_vm_call);
     register_word(vm, "VM-HEAT", mama_word_vm_heat);
     /* USE (FABRIC-2.md §F.24): not console-specific -- any VM can