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/nav-geography.sx

@@ -14,3 +14,13 @@
 
 (define marshes-examples-nav-items (list {:href "/sx/(geography.(marshes.hypermedia-feeds))" :label "Hypermedia Feeds State"} {:href "/sx/(geography.(marshes.server-signals))" :label "Server Writes to Signals"} {:href "/sx/(geography.(marshes.on-settle))" :label "sx-on-settle"} {:href "/sx/(geography.(marshes.signal-triggers))" :label "Signal-Bound Triggers"} {:href "/sx/(geography.(marshes.view-transform))" :label "Reactive View Transform"}))
 
+
+(define semantics-nav-items
+  (list
+    (dict :label "Capabilities" :href "/sx/(geography.(capabilities))"
+          :summary "Abstract evaluation contexts — what an expression can do, not where it runs.")
+    (dict :label "Modules" :href "/sx/(geography.(modules))"
+          :summary "The (use) form — declaring dependencies for documentation and static analysis.")
+    (dict :label "Eval Rules" :href "/sx/(geography.(eval-rules))"
+          :summary "Machine-readable SX semantics — 35 rules as queryable data.")))
+