Make Stage 0 the stages index: reader lands on 'now' with the full roadmap at a glance

- Merge stage-0-now.org content into stages/_index.org (now titled 'Stage 0: Now')
- Delete stage-0-now.org
- Add stage overview table to the top of the page (8 stages, what changes, threat eliminated)
- Update all cross-references from old stage-0 UUID to stages _index UUID
- Regenerate ID map to reflect the deletion
- Also fix page-list shortcode: .ByTitle -> .ByWeight so subpage listing matches sidebar order
- Rebuild: 147 files, 0 errors

ideas/lisp-game-engine-substrate.org

@@ -1,4 +1,5 @@
 :PROPERTIES:
+:ID:       f467ce16-1861-4ebd-96ed-b52fea909515
 :CREATED:  [2026-06-05 Fri]
 :END:
 #+title: Lisp as Game Engine Substrate