§H.12 step 4: Hera registers as session zero; punch list to checkboxes

Rewired stadium_birth_hera() to admit unpinned then register through
session_register()/session_set_pinned() instead of setting
STADIUM_FLAG_PIN directly on the candidate header. Self-referential
parent (vm_uuid_hera(), vm_uuid_hera()), matching capsule_run.h's
parent_vm_id == vm_id root convention. Soft-fail, non-fatal, if
session_register() fails -- Hera's actual Stadium admission is what the
patron-zero invariant is about. Wired session_boot_init() into
kernel_main.c right after stadium_boot_init(), before stadium_birth_hera().

Also converted §H.12's punch list from bold "DONE" markers to this
document's established - [ ]/- [x] checkbox convention (already used
throughout §A), for consistency.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64), no soft-fail message
on any arch, Hermes/Artemis births unaffected.

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

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