Phase C: distributed messaging capsule + idle-loop pump

Extract the messaging vocabulary (arenas, MSG-*/CH-*/MBR-* words) out of
capsules/hermes/init.4th into a new shared capsules/common/messaging.4th
that Hermes and Artemis each load at birth, giving every VM its own
private MSG-ARENA/CH-ARENA instead of only Hermes having one. Hermes
stays the owner of the one real, canonical COMMON-CH; Artemis subscribes
into it via VM-EXEC at her own birth, and Hermes proactively subscribes
Hera (idx 0) since Hera always exists first.

Hera does NOT get her own copy: register_child_vm_words()'s own doc
comment explains why the STADIUM-* primitives common:messaging.4th
depends on are deliberately never registered in her dictionary (keeps
her dict_hash off item 4.1's baseline). Confirmed live by loading it
into her dictionary anyway first -- every colon-definition referencing
an unregistered Stadium primitive was silently dropped (MSG-HEAT@/!,
CH-HEAT@/!, MSG-COOL-ALL, MSG-TICK all missing after boot). Reverted
that path; she orchestrates via BIRTH/VM-EXEC/VM-CALL instead.

Added capsule_vm_registry_get_by_index() (capsule_birth.c/.h) for
registry enumeration by birth-order position, and a pump in repl.c's
existing idle hook that walks every live VM once per idle beat and
VM-EXECs MSG-TICK into each one except Hera's own entry.

Verified clean (no UNKNOWN WORD / VM-EXEC errors after birth) on all
three architectures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019ZGkimpfyh63EZyRkNbkPD

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