feat(build): the stick and the data stick, Artemis formatted and signed at build

The stick's increment 3, Task 1 (docs/v4.0.0/plans/2026-10-27-stick-3-images.md;
STICK.md 4). make boot_image TARGET=PC now writes the 2 GiB stick: ESP 64 MiB
(BOOTX64.EFI, startup.nsh, starforth.cfg), StarshipOS identity 500 MiB (zeros),
StarshipOS Artemis to the end. make -f kernel/Makefile ARCH=<arch> datastick
writes the 1.5 GiB data stick, identity and Artemis, fresh every time. Sizes in
kernel/Makefile; every partition on whole MiB.

tools/mkartemis formats and signs an Artemis partition at build with the
kernel's own code -- v3's block_subsystem.c (blk_subsys_confirm_format) and
artemis_sig.c (artemis_sig_genesis_stamp) -- through a blkio device over a byte
range of the image; --check reads one back the same way (read-only). disk_uuid
from getrandom(2): each image its own. mkdiskimage.sh gains --starship,
--mkartemis, --esp-mib; Milk-V and the Pi images are unchanged. The kernel is
not changed.

Red: scripts/check_images.sh FAIL (no datastick, the PC image one partition).
Green: 23/23. Mutation: --check on a range shifted by one block fails.
Accepted: suite EXIT 0; 133146, 133417, 133815, POST 538/538, dict_hash
0xfec8c47cfc6f437f; test-32, 0 failures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

disk/README.md

@@ -160,6 +160,15 @@ carrying timestamp noise in git history.
   `thumbdrives/zuse-thumb-ident.img`, which stay blank (that call is Captain
   Bob's).
 
+- **The StarshipOS stick and data stick are not here** (2026-10-27,
+  `docs/v4.0.0/STICK.md` §4, §9): they are build output, made fresh by
+  `make boot_image TARGET=PC` (`build/boards/pc/lithos-pc.img`: ESP, StarshipOS
+  identity, StarshipOS Artemis) and `make -f kernel/Makefile ARCH=<arch> datastick`
+  (`build/<arch>/kernel/datastick.img`: identity and Artemis), Artemis formatted
+  and signed at build by `tools/mkartemis.c`; `scripts/check_images.sh` checks
+  both. The convention below is for fixtures kept as they are, not images the
+  build writes every time.
+
 **Convention, standing as of 2026-08-22: every virtual disk/thumb-drive image
 used for testing — Artemis persistence disks above, and USB Mass Storage
 backing images alike — lives in this directory and is a tracked, committed