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>

Kconfig.kernel

@@ -42,6 +42,18 @@ config HEARTBEAT_DOE_LOG
 	  (experiments/bare_metal/). On by default; the DoE tooling expects
 	  this log to be present.
 
+config STADIUM_MAX_VM_COUNT
+	int "Outer Stadium VM population bound (STADIUM_MAX_VM_COUNT)"
+	default 4
+	help
+	  Hard bound on live VMs in the outer Stadium (FABRIC.md item 1.5).
+	  Birth is refused once this many VMs are LIVE simultaneously; a
+	  dead or stillborn VM's registry slot does not count against the
+	  bound. Default of 4 matches Tripod's currently-known topology
+	  (Hera + two Hermes instances + Artemis) -- an explicit placeholder
+	  pending a DoE campaign to find an idealized default (item 5.1),
+	  not a padded estimate.
+
 endif # STARFORTH_VARIANT_KERNEL
 
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