Regression gap: commit a2fd8d6 (off-by-one excluding the last bulk-extended block from FSM vacuum) shipped with no test. Add tests exercising bulk extension at sizes straddling FSM slots-per-page and VM heap-blocks-per-page boundaries (±1), asserting free space is findable via pg_freespacemap and VM state via pg_visibility.
Regression gap: commit a2fd8d6 (off-by-one excluding the last bulk-extended block from FSM vacuum) shipped with no test. Add tests exercising bulk extension at sizes straddling FSM slots-per-page and VM heap-blocks-per-page boundaries (±1), asserting free space is findable via pg_freespacemap and VM state via pg_visibility.