Advance one bounded segment bootstrap page

Internal Segment Sync expansion contract: evaluates at most one keyset page, returns those exact member ids, and inserts the same ids into an unfinished ackDB snapshot in one short transaction. A downstream ledger consumes this response directly, so initial sync no longer runs the segment filter once for the snapshot and again for the ledger. Replaying an already covered after page is idempotent; a cursor ahead of durable progress is rejected. Existing bootstrapped segments use the same endpoint as a live read for additional connections. Pass the first page's filterHash as expectedFilterHash on later pages; a mid-seed filter edit returns 409 FILTER_CHANGED before writing a mixed page.