starkernel: item 2.2 complete -- bound the VM registry

Wires the birth-refusal check into capsule_birth_baby(): calls
vm_registry_live_count() (added in the prior WIP commit) between
capsule validation and vm_registry_alloc(), returning the new
CAPSULE_RUN_ERR_FLEET_FULL and logging via
capsule_parity_log_birth_failed() before any EMBRYO registry slot is
consumed.

Also fixes a gap in that WIP commit: STADIUM_MAX_VM_COUNT was only
ever defined via a Kconfig .config-driven -D flag, with no fallback
default the way every sibling knob in starforth_config.h has -- a
build with no .config present (this one) failed with the macro
undeclared. Added STARFORTH_CONFIG_STADIUM_MAX_VM_COUNT_DEFAULT (4,
matching Kconfig.kernel) following the existing HEARTBEAT_INFERENCE_FREQUENCY
pattern exactly.

All three architectures boot clean to ok> with dict_hash=0x3d4e1daf289da94f,
matching the item-0.10/2.1 baseline. FABRIC.md item 2.2 checked off.

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

include/starforth_config.h

@@ -70,6 +70,7 @@
 #define STARFORTH_CONFIG_HEARTBEAT_THREAD_ENABLED_DEFAULT 1
 #define STARFORTH_CONFIG_HEARTBEAT_TICK_NS_DEFAULT 10000ULL
 #define STARFORTH_CONFIG_HEARTBEAT_INFERENCE_FREQUENCY_DEFAULT 1000
+#define STARFORTH_CONFIG_STADIUM_MAX_VM_COUNT_DEFAULT 4
 #define STARFORTH_CONFIG_HEARTBEAT_CHECK_FREQUENCY_DEFAULT 256
 #define STARFORTH_CONFIG_HEARTBEAT_WINDOW_TUNING_FREQUENCY_DEFAULT 1000
 #define STARFORTH_CONFIG_HEARTBEAT_SLOPE_VALIDATION_FREQUENCY_DEFAULT 5000
@@ -155,6 +156,10 @@
 #define HEARTBEAT_INFERENCE_FREQUENCY STARFORTH_CONFIG_HEARTBEAT_INFERENCE_FREQUENCY_DEFAULT
 #endif
 
+#ifndef STADIUM_MAX_VM_COUNT
+#define STADIUM_MAX_VM_COUNT STARFORTH_CONFIG_STADIUM_MAX_VM_COUNT_DEFAULT
+#endif
+
 #ifndef HEARTBEAT_CHECK_FREQUENCY
 #define HEARTBEAT_CHECK_FREQUENCY STARFORTH_CONFIG_HEARTBEAT_CHECK_FREQUENCY_DEFAULT
 #endif