Phase 8 C (4/n): metadata fence allocator integration + zone I/O

Corrected meta_fence_blocks units from "Forth 1 KiB blocks" to 4 KiB
devblocks (matching bam_devblocks/reloc_devblocks) before anything
depended on the original meaning -- a clean fix, not a migration. This
let the fence fold directly into compute_totals_from_B()'s existing
payload4k formula (total_devblocks - 1 - B - R - F) instead of a
separate user_blocks subtraction: total_blocks/user_blocks/free_blocks
all shrink correctly for free, in both the fresh-format and reload
paths, from one formula change.

New blk_meta_zone_read()/blk_meta_zone_write() -- raw, unpacked 4 KiB
devblock I/O, same shape as the header/BAM/reloc-table regions,
addressed by devblock_from_top counting down from the device's last
physical devblock. Refuses rather than clamps if the index exceeds the
on-disk meta_fence_blocks. C-only, no FORTH word wraps either -- same
discipline as vm_zuse_cert_install(), which will be this zone's first
real tenant.

Verified independently at every step, never trusting the kernel's own
report: capacity math cross-checked against a from-scratch Python
recomputation of the same formula (exact match); accessor correctness
via a temporary probe (written/run/captured/reverted) that wrote a
known pattern and read it back, then independently confirmed via a raw
read of the disk image at the exact expected physical byte offset.
Full 3-arch acceptance boot against the real, untouched disk/artemis.img,
probe code fully reverted -- clean, conservation intact.

Still open: wiring vm_zuse_cert_install() to actually persist through
these accessors, and the MINT word itself. Documented in FABRIC-3.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd

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