Add VM-HEAT primitive: groundwork for a compudynamic turn-attractor (FABRIC-3.md §XIX)

Prompted by reading the std79-doe K report: checked whether the campaign's
"real cross-VM dispatch load" was actually concurrent or strictly
serialized. It's serialized at two levels -- VM-EXEC's vm_interpret(target,
...) is a direct synchronous C call (Hera fully blocked until it returns),
and even the background physics tick (vm_tick(), vm_runtime.c) is driven
by each VM's own execution loop, so idle identities accrue zero ticks
between their own turns. K's perfect conservation (FABRIC-3.md §XVIII)
verifies sequential per-VM accounting correctness, not concurrent-access
safety, since there was never concurrent access to test.

Agreed direction: fix this without a scheduler, by reusing the same
least-dense-candidate judgment stadium_admit() already trusts for
eviction, applied to "whose turn is next" instead of "who gets evicted" --
a fleet-level turn-attractor giving the next turn to whichever live
identity currently has the lowest execution_heat_q48, no fixed round-robin,
no priorities, no preemption. Lives beside Stadium in
capsule_vm_physics.c (already the fleet-level consumer of Stadium
primitives, e.g. the K mechanism itself), not inside stadium.c ("the
floor" -- residency/eviction, a different concern from turn order) and
not a new subsystem.

This pass lands only the primitive the mechanism needs: VM-HEAT
( c-addr u -- heat-q48 ), pushing a named VM's current
execution_heat_q48 via vm_physics_heat_of() -- previously C-internal
only (doe_log_heat_by_name(), doe_log.c), never exposed to FORTH. Silent
0 on an unknown/dead name (no print/error), since a turn-attractor
scanning many candidates every turn shouldn't have to filter console
noise for names that simply aren't live. Registered everywhere
VM-EXEC/VM-CALL already are. Builds clean on all three architectures;
live-tested on amd64: Hera -> 65452, Hermes -> 43, unknown name -> 0, no
faults.

The turn-attractor loop itself (std79-doe.fth's trial ordering) is not
yet built -- next step, not done here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

src/starkernel/capsule/mama_forth_words.c

@@ -865,6 +865,53 @@ static void mama_word_vm_call(VM *vm)
     }
 }
 
+/**
+ * @brief VM-HEAT ( c-addr u -- heat-q48 )
+ * Push the named VM's current execution_heat_q48 (Q48.16, 0 if the VM is
+ * unknown/dead), via vm_physics_heat_of() -- previously C-internal only
+ * (doe_log_heat_by_name(), doe_log.c), never exposed to FORTH. Added
+ * 2026-09-12 for a compudynamic turn-attractor (FABRIC-3.md §XIX): "whose
+ * turn is next" decided by comparing live candidates' own current heat via
+ * this word, the same density-comparison judgment stadium_admit() already
+ * makes for eviction, rather than a fixed round-robin order. Silent on an
+ * unknown name (returns 0, does not print/error) -- callers scanning many
+ * VM names each turn to find the coolest one should not have to filter
+ * console noise for names that are simply not currently live.
+ */
+static void mama_word_vm_heat(VM *vm)
+{
+    char        vm_name[VM_NAME_MAX];
+    uint32_t    i;
+    cell_t      vm_u, vm_caddr;
+    const char *src;
+    VMRegistryEntry entry;
+
+    if (vm->dsp < 1) { vm->error = 1; return; }
+
+    vm_u     = vm_pop(vm);
+    vm_caddr = vm_pop(vm);
+
+    if (vm_u <= 0 || (uint32_t)vm_u >= VM_NAME_MAX) {
+        vm_push(vm, 0);
+        return;
+    }
+
+    {
+        const uint8_t *p = vm_ptr(vm, (vaddr_t)vm_caddr);
+        if (!p) { vm->error = 1; return; }
+        src = (const char *)p;
+    }
+    for (i = 0; i < (uint32_t)vm_u; i++) vm_name[i] = src[i];
+    vm_name[vm_u] = '\0';
+
+    if (capsule_vm_find_by_name_nocase(vm_name, &entry) != 0) {
+        vm_push(vm, 0);
+        return;
+    }
+
+    vm_push(vm, (cell_t)vm_physics_heat_of(entry.vm_id));
+}
+
 /**
  * @brief MINT ( -- ok? )
  * Mint a fresh identity onto the currently attached USB drive
@@ -1586,6 +1633,7 @@ void register_mama_forth_words(VM *vm)
     register_word(vm, "VM-STEP", mama_word_vm_step);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
     register_word(vm, "VM-CALL", mama_word_vm_call);
+    register_word(vm, "VM-HEAT", mama_word_vm_heat);
     register_word(vm, "CAPSULE-TEST", mama_word_capsule_test);
 
     /* Create and switch to MAMA vocabulary */
@@ -1623,6 +1671,7 @@ void register_mama_forth_words(VM *vm)
     register_word(vm, "VM-STEP", mama_word_vm_step);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
     register_word(vm, "VM-CALL", mama_word_vm_call);
+    register_word(vm, "VM-HEAT", mama_word_vm_heat);
     register_word(vm, "CAPSULE-TEST", mama_word_capsule_test);
     register_word(vm, "EXEC", mama_word_exec);
 
@@ -1837,6 +1886,7 @@ void register_child_vm_words(VM *vm)
     register_word(vm, "EXEC",    mama_word_exec);
     register_word(vm, "VM-EXEC", mama_word_vm_exec);
     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
      * redirect the physical REPL to any other VM it has ACL access to
      * (BINDSTEP re-verifies on every call, §F.9), including a console