Explainer trilogy: drafted, built, and shipped in two formats

Three-part LTP explainer series in explainers/, replacing the earlier single story/ piece as the project's introductory narrative. Each piece went through a script pass (draft → self-critique → independent fresh-agent critique with no authorship context → revision), then a visual build.

• Added skills/scrollable-explainer/ — general-purpose write-up of the scroll-driven ("scrollytelling") explainer form, with a full measured teardown of a ProPublica piece as the worked case study.
• Wrote and built three pieces: The Wrong Queue (01-bottleneck), The Arrows Nobody Checks (02-thinking-made-visible), Five Shapes (03-five-shapes).
• Each part now ships in two forms from the same finalized prose:
  • article.md — static markdown with inline SVG figures, no <script> (matches the story/draft.md convention)
  • index.html — scroll-driven animated version, via shared explainers/scroller.js / scroller.css
• explainers/index.html lists both formats per part.
• Mobile portrait layout for piece 1's five-station diagram (pieces 2–3 still pending).
• explainers/NEXT.md — checkpoint with open questions and a prioritized follow-up list. Tracked in issue #6.

Known gaps: pieces 2–3 need the same mobile pass as piece 1; piece 3's HTML page renders empty with JavaScript disabled; the scroller uses an ad hoc palette rather than the real Organic design tokens; story/'s fate is undecided.