Channel table + common channel, inert (B1) -- FABRIC-3.6.md task 3.2

Adds SkHermesChannel: a channel is an index into a boot-time,
stadium_max_vm_count()-sized table (same sizing pattern task 3.1
established for the switch table -- no separate numeric rule was ruled
for this table, so task 3.1's bound is extended directly, flagged as
such rather than restated as a new ruling). No name field, mirroring
messaging.4th's own nameless CH-ARENA.

The common channel (index 0) is created at boot and permanent. Hera
subscribes explicitly in kernel_main.c (she is the one VM never born
through capsule_birth_baby()); every other VM -- Tripod fleet and
future WIREBIND identities alike -- subscribes inside
capsule_birth_baby() itself, the single choke point every other birth
already passes through.

Inert: no publish, no dispatch, no ACK/NACK, no ACL hook (tasks 3.3,
3.6, 3.7). Verified live on all three architectures: channel table
sized to 50/202/50 slots (matching switch-signal's own per-arch
sizing), common-channel fleet self-test confirms all four Tripod
members are members, and a synthetic create/subscribe/unsubscribe/
destroy round-trip against a private topic passes, including refusing
to destroy the common channel.

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

src/starkernel/capsule/capsule_birth.c

@@ -38,6 +38,7 @@
 #include "starkernel/session.h" /* session_register()/session_set_pinned() -- FABRIC-2.md §H.12 step 5 */
 #include "starkernel/timer.h" /* timer_calibration_record()->vm_mode -- FABRIC-2.md §I.5 CONTRIB trust tier */
 #include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 4 -- capsule_vm_force_reap() */
+#include "starkernel/vm/kernel_hermes.h" /* FABRIC-3.6.md task 3.2 -- sk_hermes_channel_subscribe() */
 #include "word_source/include/framebuffer_words.h" /* FABRIC-3.6.md task 1.8 -- register_framebuffer_words() */
 #include "vm.h"
 #include "platform_alloc.h"
@@ -857,6 +858,16 @@ CapsuleRunResult capsule_birth_baby(
     entry->state           = VM_STATE_LIVE;
     entry->birth_dict_hash = dict_hash;
 
+    /* FABRIC-3.6.md task 3.2 (B1 / FABRIC-3.5.md §XLV.1): every VM joins
+     * the common channel at birth. This is the single choke point every
+     * non-Hera birth passes through (BIRTH, WIREBIND identities, DoE
+     * workload births alike) -- Hera herself is subscribed explicitly in
+     * kernel_main.c, the one birth path that never reaches here. Soft
+     * failure, same posture as every other call in this function -- a
+     * refused subscription (table exhausted) is logged nowhere special
+     * and does not fail the birth itself. */
+    (void)sk_hermes_channel_subscribe(SK_HERMES_CHANNEL_COMMON, vm_id);
+
     capsule_parity_log_birth(vm_id, cap->capsule_id, cap->content_hash, dict_hash);
 
     /* §H.12 step 11: applied AFTER dict_hash/parity logging above, not