Memory-focused zset rework: non-power-of-two member index (flat ~49 B/member,
~37% of Redis RSS at any size), tunable member-index growth (--member-index-growth,
default 2^0.25), and GOBLIN.OPTIMIZE key [density] in-place compaction. Faster on
every supported op vs Redis. Backward-compatible pre-1.0 minor release.