Capabilities: rename from Semantics, add server/client fetch demos

- Removed duplicate one-liner sx-nav-tree (leftover from before refactor)
- Renamed "Semantics" → "Capabilities" in nav tree — direct geography
  entry, no subsection
- Added "Where code evaluates" section with three demos:
  - Server-side fetch (SSR with io capability)
  - Client-side fetch (island with reactive signals)
  - Same component both ways (isomorphic evaluation)
- Shows how capabilities abstract over location — same code runs
  server or client depending on the evaluation context

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

sx/sx/nav-tree.sx

@@ -76,7 +76,8 @@
           (dict
             :href "/sx/(geography.(cek))"
             :label "CEK Machine"
-            :children cek-nav-items)))
+            :children cek-nav-items)
+          (dict :href "/sx/(geography.(capabilities))" :label "Capabilities")))
       (dict
         :href "/sx/(language)"
         :label "Language"
@@ -154,7 +155,6 @@
               (= (get child "href") path)
               (has-descendant-href? child path)))
           children)))))
-(define sx-nav-tree {:href "/sx/" :children (list {:href "/sx/(geography)" :children (list {:href "/sx/(geography.(reactive))" :children reactive-islands-nav-items :label "Reactive Islands"} {:href "/sx/(geography.(hypermedia))" :children (list {:href "/sx/(geography.(hypermedia.(reference)))" :children reference-nav-items :label "Reference"} {:href "/sx/(geography.(hypermedia.(example)))" :children examples-nav-items :label "Examples"}) :label "Hypermedia Lakes"} {:href "/sx/(geography.(scopes))" :summary "The unified primitive beneath provide, collect!, spreads, and islands. Named scope with downward value, upward accumulation, and a dedup flag." :label "Scopes"} {:href "/sx/(geography.(provide))" :summary "Sugar for scope-with-value. Render-time dynamic scope — the substrate beneath spreads, CSSX, and script collection." :label "Provide / Emit!"} {:href "/sx/(geography.(spreads))" :summary "Child-to-parent communication across render boundaries — spread, collect!, reactive-spread, built on scopes." :label "Spreads"} {:href "/sx/(geography.(marshes))" :children marshes-examples-nav-items :summary "Where reactivity and hypermedia interpenetrate — server writes to signals, reactive transforms reshape server content, client state modifies how hypermedia is interpreted." :label "Marshes"} {:href "/sx/(geography.(isomorphism))" :children isomorphism-nav-items :label "Isomorphism"} {:href "/sx/(geography.(cek))" :children cek-nav-items :label "CEK Machine"} {:href "/sx/(geography.(capabilities))" :children semantics-nav-items :label "Semantics"}) :label "Geography"} {:href "/sx/(language)" :children (list {:href "/sx/(language.(doc))" :children docs-nav-items :label "Docs"} {:href "/sx/(language.(spec))" :children specs-nav-items :label "Specs"} {:href "/sx/(language.(spec.(explore.evaluator)))" :label "Spec Explorer"} {:href "/sx/(language.(bootstrapper))" :children bootstrappers-nav-items :label "Bootstrappers"} {:href "/sx/(language.(test))" :children testing-nav-items :label "Testing"}) :label "Language"} {:href "/sx/(applications)" :children (list {:href "/sx/(applications.(sx-urls))" :label "SX URLs"} {:href "/sx/(applications.(cssx))" :children cssx-nav-items :label "CSSX"} {:href "/sx/(applications.(protocol))" :children protocols-nav-items :label "Protocols"} {:href "/sx/(applications.(sx-pub))" :label "sx-pub"} {:href "/sx/(applications.(reactive-runtime))" :children reactive-runtime-nav-items :label "Reactive Runtime"}) :label "Applications"} {:href "/sx/(tools)" :children tools-nav-items :label "Tools"} {:href "/sx/(etc)" :children (list {:href "/sx/(etc.(essay))" :children essays-nav-items :label "Essays"} {:href "/sx/(etc.(philosophy))" :children philosophy-nav-items :label "Philosophy"} {:href "/sx/(etc.(plan))" :children plans-nav-items :label "Plans"}) :label "Etc"}) :label "sx"})
 
 (define
   find-nav-match