backup: correct the target_not_empty rule, the store root ancestor rule and a remove-target pointer #942

Some backup texts on main state rules that the code does not follow. #897 makes the same corrections in the upgrade manual.

When a restore fails as target_not_empty

The reconciler fails a store-backed restore with no recorded progress as target_not_empty only when the target holds more items than the backup's item_count (exceeds_backup_total in crates/engine/src/backup/reconciler.rs). At or below that count, the takeover arm resumes the restore and applies the first data file again. While a backup command holds the maintenance claim, the reconciler fails such a restore as OWNER_DEAD and removes its target.

These texts say otherwise:

• crates/engine/src/backup/maintenance.rs, module doc, lines 39 to 43: on MongoDB and Cassandra, a crash before the first progress record ends in target_not_empty.
• crates/app/src/cmd_backup.rs, module doc, lines 118 to 121: the same claim.
• crates/app/src/cmd_backup.rs lines 76 to 78: a record with no recorded progress over a target that holds items fails as target_not_empty. The code requires more items than the backup holds.
• crates/engine/src/backup/reconciler.rs lines 71 to 74 and 822 to 824: the same statement.

The directories above a filesystem root

crates/backup-store/src/fs.rs refuses an ancestor directory that group or others can write to, unless the directory carries the sticky bit. These texts name only others and leave out the sticky bit:

• docs/manuals/05-admin-guide.md line 168: "sit under directories owned by root or that user which others cannot write to".
• The default-root warning in crates/server/src/backup_store.rs (warn_default_root_unusable): "under directories owned by root or this user that others cannot write to".

docs/manuals/14-backup-and-restore.md states the rule as the code applies it.

The remove-target pointer

docs/manuals/14-backup-and-restore.md lines 479 and 480 send the reader to "the admin guide's backup section" for extenddb backup remove-target. The admin guide has no such section. It describes the command in its [backup.store] section.