docs: align state-file terminology with crawler/scanner/tier model

docs: align state-file terminology with crawler/scanner/tier model

Each [scan.extent.NAME] section is a scanner (a size tier); each scanner
has one crawler that enumerates extents and tracks the cursor recorded in
the section.  The previous text had the crawler "perform the scanning",
which inverted the roles.

Apply the same model to subvol sections: one scanner per subvol with a
crawler tracking the cursor, and drop the singular-engine "the subvol
scanner creates sections" framing in favour of describing the sections
appearing and disappearing with the subvolumes.

Assisted-by: Claude-Code:claude-opus-4-8