memba_feed_v1 package

SweepTombstones hard-removes up to `limit` soft-deleted posts, reclaiming AVL storage so post+delete spam cannot accrete permanent state. Permissionless by design (state-shrink hygiene primitive, not a refund path). Bounded + idempotent: keep `limit` small (1-10); re-running drains the next batch and stops at 0. Returns the number of posts swept.

For a swept post that was a PARENT, its surviving replies' byParent links (`padID(parent):padID(child)`) would otherwise be orphaned under an id no longer in `posts` — a permanent leak plus a read inconsistency (replies still enumerable, getReplyCount() disagreeing). So the sweep range-deletes that byParent prefix; the reply posts themselves stay as standalone live content (their parent was removed by its own author). Bounded because the live reply set per post is capped at MaxRepliesPerPost — but a reply-heavy parent makes one sweep step do up to that many removes, so keep `limit` small (1) when draining known reply-heavy tombstones.