WIP: item 2.2 -- bound the VM registry (not complete, do not check off)

Partial work toward FABRIC.md punch list item 2.2. Adds the
STADIUM_MAX_VM_COUNT Kconfig symbol (default 4, per item 1.5) wired
through Makefile.starkernel, a new CAPSULE_RUN_ERR_FLEET_FULL result
code, and a vm_registry_live_count() helper in capsule_birth.c that
counts LIVE VMs only (distinct from the existing monotonic
vm_registry_count, which never decrements on death).

NOT YET DONE: nothing calls vm_registry_live_count() yet -- the actual
birth-refusal check is not wired into capsule_birth_baby(). Not built,
not boot-tested. FABRIC.md's item 2.2 checkbox is deliberately left
unchecked; this commit exists only to save in-progress work before a
pause, not to claim the item complete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Makefile.starkernel

@@ -277,6 +277,7 @@ $(eval $(call kconfig_int,INITIAL_DECAY_SLOPE_Q48,21845))
 $(eval $(call kconfig_int,DECAY_MIN_INTERVAL,500))
 $(eval $(call kconfig_int,DECAY_RATE_PER_US_Q16,1))
 $(eval $(call kconfig_int,HEARTBEAT_INFERENCE_FREQUENCY,1000))
+$(eval $(call kconfig_int,STADIUM_MAX_VM_COUNT,4))
 $(eval $(call kconfig_int,SSM_ENTROPY_HIGH_THRESHOLD,0.75))
 $(eval $(call kconfig_int,SSM_CV_HIGH_THRESHOLD,0.15))
 $(eval $(call kconfig_int,SSM_TEMPORAL_DECAY_THRESHOLD,0.5))
@@ -320,7 +321,7 @@ VM_FEATURE_FLAG_VARS := \
     HEARTBEAT_TICK_NS HEARTBEAT_INFERENCE_FREQUENCY \
     HEARTBEAT_CHECK_FREQUENCY HEARTBEAT_WINDOW_TUNING_FREQUENCY \
     HEARTBEAT_SLOPE_VALIDATION_FREQUENCY SK_PARITY_DEBUG \
-    EMERGENCY_CONSOLE_ENABLED \
+    EMERGENCY_CONSOLE_ENABLED STADIUM_MAX_VM_COUNT \
     SSM_ENTROPY_HIGH_THRESHOLD SSM_CV_HIGH_THRESHOLD \
     SSM_TEMPORAL_DECAY_THRESHOLD SSM_TEMPORAL_DECAY_LOW_THRESHOLD \
     SSM_HYSTERESIS_TICKS \