Fix Artemis persist-read: reformat corrupted disk/artemis.img

Chases down the amd64/aarch64/riscv64 "FAIL: persist-read" that
capsules/artemis/init.4th's ART-READ-TEST self-test has been reporting
in every boot log in this repo's history.

Root cause: not a code bug. disk/artemis.img had been stuck in a
corrupted state (valid LithosAnanke magic header, but data not matching
what ART-READ-TEST expects) since before this repo's own git history
begins -- already broken at the initial commit, carried over from the
pre-split monorepo. The FAIL was accurate: it correctly reported bad
data, not bad code.

Verified via a fresh disk/artemis-debug-roundtrip.img: format ->
self-test PASS -> write-test PASS -> reboot -> resume -> PASS:
persist-read, confirmed 3 times in a row. The write/read/persist code,
free map, block allocator, and C-level block subsystem cache/writeback
logic are all correct.

Fix: blanked disk/artemis.img and let a normal boot format + write-test
it fresh, then verified PASS: persist-read on amd64, aarch64, AND
riscv64 against the same reformatted image -- confirming the arch-neutral
on-disk format works cross-arch too (a boot on one architecture writes
data the other two can correctly read back).

disk/artemis-debug-roundtrip.img is kept as a regression fixture, already
in a known-good passing state -- a future break here is a real
regression, not fixture rot like artemis.img turned out to be.

disk/README.md: documented both images' state, and corrected a stale
claim that these images are managed via scripts/rundisk.sh -- that
script actually targets a separate, currently-unused disks/ (plural)
directory for the hosted VM's --disk-img= flag, not this kernel-QEMU
disk/ (singular) one.

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

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