§H.12 step 12: BMAPFMT field layout in blk_meta_t

Replaced the old 40-byte owner_id/permissions/acl_block/signature[2]
with owner_fp[8]/acl_allow/acl_ttl (u32)/acl_reserved[3]/reserved_future,
matching the decided §F.4/§H.6 layout.

Found a pre-existing bug via a real offsetof/sizeof compile check
(not hand math, per this step's own instruction): sizeof(blk_meta_t)
was already 344, not the 341 its own BLK_META_PER_BLOCK constant and
"341-byte slice" comment claimed -- harmless since that constant has
zero callers anywhere. New size after this edit's own alignment
padding is 336. Added a _Static_assert matching blk_volume_meta_t's
existing precedent, and fixed the stale comment to point at it.
BLK_META_PER_BLOCK itself untouched -- unused, out of scope.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64).

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

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