L1 guide: OTA and A/B

The two headers

• kernel/ota/ota_ab.h (21 declarations): slot discovery, flags, slot state, install-onto-slot, rollback - the numbered +7.
• kernel/ota/ota_update.h (12 declarations): the chain - check, download, verify (SHA256 + signature), install, set_boot, get_status, rollback.

The L1 mirror

l1/l1_ext.h mirrors the chain for module code: l1_ext_check_update(_async), l1_ext_update_check_pkg, l1_ext_update_download, l1_ext_update_verify, l1_ext_update_install, l1_ext_update_rollback, l1_ext_update_set_boot, l1_ext_update_get_status.

Using it from a module

The canonical flow (what the update command runs):

l1_ext_check_update(&info);            // manifest from /etc config
l1_ext_update_download(url, path);
l1_ext_update_verify(path, sha256);
l1_ext_update_install(path, "B");      // inactive slot
// next boot flips; l1_ext_update_rollback() flips back

The l1_ext_update_info_t.version[64]/changes[256] mirror ota_update_info_t exactly (the 32->64 widening was a real bug: 32 truncated the 44-char official version string).

In-guest tests

ab_partition_test, the update_*_test battery, checkupdate_test.