Add Philosophy section with Wittgenstein, Dennett, and S-Existentialism essays

New top-level Philosophy section in sx-docs. Moved SX Manifesto and Strange
Loops from Essays, added three new essays: SX and Wittgenstein (language games,
limits of language, fly-bottles), SX and Dennett (real patterns, multiple drafts,
intentional stance), and S-Existentialism (existence precedes essence, bad faith,
the absurd). Updated all cross-references and navigation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/content/pages.py

@@ -72,7 +72,7 @@ ESSAYS_NAV = [
     ("On-Demand CSS", "/essays/on-demand-css"),
     ("Client Reactivity", "/essays/client-reactivity"),
     ("SX Native", "/essays/sx-native"),
-    ("The SX Manifesto", "/essays/sx-manifesto"),
+    ("The SX Manifesto", "/philosophy/sx-manifesto"),
     ("Tail-Call Optimization", "/essays/tail-call-optimization"),
     ("Continuations", "/essays/continuations"),
 ]