Stage C-a: work law — running VM charged at each of its own beats, with carry (FABRIC-3.8 §3.13.2)

vm_physics_charge(): per-beat slope into a per-VM carry, whole units pulled
from the rest of the live fleet in proportion to heat (conservative
vm_physics_transfer), rounding shortfall kept in the carry, no fleet_window
sample. Called from vm_tick() only for the switch-current VM. Closes G9:
a VM running its own mail on its own stack now heats up while it works.

Probe (reverted): baseline Hestia heat 0->0->0 over 71 beats (FAIL, proves
G9); with charge 0->11->23 at beats 15->51->86, identical on amd64/aarch64/
riscv64, conserved, PASS. Acceptance: dict_hash unchanged, WIREBIND relay 3.

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

src/starkernel/vm/vm_runtime.c

@@ -51,6 +51,7 @@
 #include "../include/ssm_jacquard.h"
 #include "vm_internal.h"
 #include "starkernel/capsule_vm_physics.h"
+#include "starkernel/vm/switch.h" /* sk_vm_switch_current_vm() -- FABRIC-3.8.md §3.13.2 */
 #include "starkernel/timer.h"
 
 #include <errno.h>
@@ -140,6 +141,17 @@ void vm_tick(VM* vm)
      * (VM-FLEET-ATTRACTOR-DESIGN-20260705.md rev k). */
     vm_physics_heartbeat_tick(vm_monotonic_ns(vm));
 
+    /* FABRIC-3.8.md §3.13.2 (Stage C-a): the work law. This beat was
+     * produced by `vm`'s own execution; if `vm` is the VM physically
+     * running (switch-current), charge it one beat. A VM whose words run
+     * nested inside someone else's VM-EXEC is not switch-current and is
+     * not charged here -- that execution is already accounted by
+     * vm_physics_touch()'s dispatch law. Kernel-side only: nothing a VM
+     * executes can ask for, skip, or give back a charge. */
+    if (vm == sk_vm_switch_current_vm()) {
+        vm_physics_charge(vm->stadium_vm_id);
+    }
+
     /* Plugin 2: System State Monitoring (Future) */
     /* vm_tick_system_monitor(vm); */