Add Semantics geography pages: capabilities, modules, eval-rules

Three new documentation pages under Geography > Semantics:

- Capabilities: abstract evaluation contexts, capability primitives,
  standard capabilities, why not phases
- Modules: the (use) form, what it enables, semantics
- Eval Rules: machine-readable rule set, sx_explain tool, rule structure

Navigation: semantics-nav-items with 3 entries, linked from geography
nav tree after CEK Machine.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -69,3 +69,9 @@
 (define philosophy (fn (slug) (if (nil? slug) (quote (~essays/philosophy-index/content)) (case slug "sx-manifesto" (quote (~essay-sx-manifesto)) "godel-escher-bach" (quote (~essays/godel-escher-bach/essay-godel-escher-bach)) "wittgenstein" (quote (~essays/sx-and-wittgenstein/essay-sx-and-wittgenstein)) "dennett" (quote (~essays/sx-and-dennett/essay-sx-and-dennett)) "existentialism" (quote (~essays/s-existentialism/essay-s-existentialism)) "platonic-sx" (quote (~essays/platonic-sx/essay-platonic-sx)) :else (quote (~essays/philosophy-index/content))))))
 
 (define plan (make-page-fn "~plans/index/plans-index-content" "~plans/" "/plan-" "-content"))
+
+(define capabilities (fn (&key title &rest args) (quasiquote (~geography/capabilities-content))))
+
+(define modules (fn (&key title &rest args) (quasiquote (~geography/modules-content))))
+
+(define eval-rules (fn (&key title &rest args) (quasiquote (~geography/eval-rules-content))))