Previously, when segments were merged during compaction, the pages from
the source segments were orphaned - they remained allocated but were no
longer referenced. This caused index bloat, with roughly 43% of space
wasted on the slack-db benchmark (831 MB → 473 MB after fix).

Changes:
- Add tp_segment_collect_pages() to collect all pages (data + page index)
  from a segment before merge
- Add tp_segment_free_pages() to record freed pages in PostgreSQL's FSM
- Call IndexFreeSpaceMapVacuum() after freeing pages so the FSM tree is
  updated and GetFreeIndexPage() can find them
- Modify allocate_segment_page() to check FSM for recycled pages before
  extending the relation
- Add FSM stats tracking for debugging (visible at DEBUG1 level)
- Demote spill/merge notices to DEBUG1 to reduce noise during builds

Testing:
- slack-db benchmark: index size reduced from 831 MB to 473 MB (43%)
- Page reuse during build: 45,026 pages reused from FSM
- All regression tests pass

… free, add DEBUG1 to merge test