name the output pool restriction for what it restricts

It is no longer about reprocessing: any recipe with several output groups is
subject to it, and the plant is only the building that happens to have one
(REQ-MAT-OUTPUT-GROUP). REQ-LOCK-REPROCESSING-POOL becomes REQ-LOCK-OUTPUT-POOL.
Four citations, all in docs -- no code cites it.

Also corrects a line REQ-LOCK-IMPLICIT still carried from before smelters had a
recipe control: their recipes are not "never shown in any UI dropdown" any more,
they are simply never gated, so the dialog offers all of them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

docs/balancing/rules.md

@@ -231,7 +231,7 @@ supporting different fleet doctrines feel structurally different to build.
   **reprocessing** (probabilistic higher intermediates, including the
   late-game input — the gamble that eventually becomes mandatory).
 - The reprocessing output pool renormalizes over implicitly unlocked items
-  (REQ-LOCK-REPROCESSING-POOL), so its output quality improves
+  (REQ-LOCK-OUTPUT-POOL), so its output quality improves
   automatically as the run progresses. **Rule:** weights are authored for
   the *fully unlocked* pool state; early-game behavior falls out of
   renormalization for free and needs no separate staging.