Segment membership: Automated atom GC

[khvn26]

Scope

• Reference-counted SegmentAtom link table, populated by ensure_atoms alongside Atom rows.
• Sweep triggers: segment delete (drop link rows; queue zero-ref atoms for deletion) and a daily background sweep for orphans introduced by other paths (% Split revision drift, operand canonicalisation drift).
• One-time backfill of the link table from the existing Atom catalogue.

Acceptance criteria

• Deleting a segment that owns unique atoms removes their bitmap blobs and Atom rows.
• Deleting a segment that shares atoms with other segments leaves the shared atoms intact.
• The daily sweep removes % Split orphans within 24h of revision.