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>

include/starkernel/capsule_run.h

@@ -54,6 +54,7 @@ typedef enum {
     CAPSULE_RUN_ERR_EXEC_FAIL,     /* Execution failed */
     CAPSULE_RUN_ERR_HASH_MISMATCH, /* Post-run hash mismatch */
     CAPSULE_RUN_ERR_STILLBORN,     /* VM birth failed */
+    CAPSULE_RUN_ERR_FLEET_FULL,    /* Outer Stadium at STADIUM_MAX_VM_COUNT (FABRIC.md item 1.5/2.2) */
 } CapsuleRunResult;
 
 /*===========================================================================