Restructure navigation around What's up / Opine and the reader's utilities

- Stories gain a required type (report | essay); all 28 classified per
  the boundary rule now stated in EDITORIAL.md
- Nav: What's up · Opine · Podcast · Book · Guides · Forum, each gated
  on real content (Forum waits on a forumUrl); logo is Home
- /whats-up (reports, newest first) and /opine (essays, editorial order)
  share a StoryArchive component with subject filters
- Static /tags/<tag> pages for subjects carried by 2+ stories; tags stay
  overlapping, never exclusive departments
- /corrections: generated from revision history (commits beginning
  'Correction:'), so the page cannot under-report
- About, RSS, Corrections, Weekly briefing move to the footer

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

EDITORIAL.md

@@ -8,6 +8,16 @@ We ask one recurring question with cheerful suspicion: when a system claims to b
 
 Stories begin with people and consequences. Technical machinery is explained in plain English. Jargon is defined when it is first used. Outrage is reserved for what the evidence supports.
 
+## Sections
+
+Stories are typed, and the boundary is a commitment readers can check: a piece pegged to a system or event in the world is a **report** (What's up); an argument that would survive the news cycle is an **essay** (Opine). Reports are expected to age into `historical` status; essays carry review dates that mean something.
+
+Subjects are overlapping tags, not exclusive departments. A story about the EU wallet may legitimately be identity, privacy, and institutions at once.
+
+## Corrections page
+
+The corrections page is generated from revision history: any commit whose message begins with `Correction:` appears there automatically. The page cannot list fewer corrections than the repository contains.
+
 ## Authorship and maintenance
 
 An author and a maintainer are different roles. Every published item identifies its author. Maintained material also names the person responsible for review.