Implements the design from the previous commit as-is: homeblocks_sig_t
(4096 bytes, magic+version+drive_uuid+timestamp+cert/blockmap offset
reservations+real hdr_crc), HOMEBLOCKS_SIG_PACK/_GET_MAGIC/_GET_VERSION
macros mirroring CAPSULE_MAGIC_PACK's bit layout, and a C99
compile-time size assertion matching stadium.h's own discipline.
Verified standalone (sizeof == 4096, clean under -std=c99 -Wall
-Wextra -Werror) -- nothing consumes this header yet, so no
functional kernel change and no 3-arch acceptance boot needed for this
step.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn
