Add continuation specs: delimited (shift/reset) and full (call/cc)

Optional bolt-on extensions to the SX spec. continuations.sx defines
delimited continuations for all targets. callcc.sx defines full call/cc
for targets where it's native (Scheme, Haskell). Shared continuation
type if both are loaded. Wired into specs section of sx-docs.

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

sx/sx/nav-data.sx

@@ -95,7 +95,9 @@
   (dict :label "SxEngine" :href "/specs/engine")
   (dict :label "Orchestration" :href "/specs/orchestration")
   (dict :label "Boot" :href "/specs/boot")
-  (dict :label "CSSX" :href "/specs/cssx")))
+  (dict :label "CSSX" :href "/specs/cssx")
+  (dict :label "Continuations" :href "/specs/continuations")
+  (dict :label "call/cc" :href "/specs/callcc")))
 
 (define bootstrappers-nav-items (list
   (dict :label "Overview" :href "/bootstrappers/")
@@ -146,7 +148,15 @@
         :desc "On-demand CSS: style dictionary, keyword resolution, rule injection."
         :prose "CSSX is the on-demand CSS system. It resolves keyword atoms (:flex, :gap-4, :hover:bg-sky-200) into StyleValue objects with content-addressed class names, injecting CSS rules into the document on first use. The style dictionary is a JSON blob containing: atoms (keyword to CSS declarations), pseudo-variants (hover:, focus:, etc.), responsive breakpoints (md:, lg:, etc.), keyframe animations, arbitrary value patterns, and child selector prefixes (space-x-, space-y-). Classes are only emitted when used, keeping the CSS payload minimal. The dictionary is typically served inline in a <script type=\"text/sx-styles\"> tag.")))
 
-(define all-spec-items (concat core-spec-items (concat adapter-spec-items browser-spec-items)))
+(define extension-spec-items (list
+  (dict :slug "continuations" :filename "continuations.sx" :title "Continuations"
+        :desc "Delimited continuations — shift/reset for suspendable rendering and cooperative scheduling."
+        :prose "Delimited continuations capture the rest of a computation up to a delimiter. shift captures the continuation to the nearest reset as a first-class callable value. Unlike full call/cc, delimited continuations are composable — invoking one returns a value. This covers the practical use cases: suspendable server rendering, cooperative scheduling, linear async flows, wizard-style multi-step UIs, and undo. Each bootstrapper target implements the mechanism differently — generators in Python/JS, native shift/reset in Scheme, ContT in Haskell, CPS transform in Rust — but the semantics are identical. Optional extension: code that doesn't use continuations pays zero cost.")
+  (dict :slug "callcc" :filename "callcc.sx" :title "call/cc"
+        :desc "Full first-class continuations — call-with-current-continuation."
+        :prose "Full call/cc captures the entire remaining computation as a first-class function — not just up to a delimiter, but all the way to the top level. Invoking the continuation abandons the current computation entirely and resumes from where it was captured. Strictly more powerful than delimited continuations, but harder to implement in targets that don't support it natively. Recommended for Scheme and Haskell targets where it's natural. Python, JavaScript, and Rust targets should prefer delimited continuations (continuations.sx) unless full escape semantics are genuinely needed. Optional extension: the continuation type is shared with continuations.sx if both are loaded.")))
+
+(define all-spec-items (concat core-spec-items (concat adapter-spec-items (concat browser-spec-items extension-spec-items))))
 
 (define find-spec
   (fn (slug)

sx/sx/specs.sx

@@ -167,7 +167,39 @@ adapter-sx.sx       depends on: render, eval
 engine.sx           depends on: eval, adapter-dom
 orchestration.sx    depends on: engine, adapter-dom
 cssx.sx             depends on: render
-boot.sx             depends on: cssx, orchestration, adapter-dom, render")))
+boot.sx             depends on: cssx, orchestration, adapter-dom, render
+
+;; Extensions (optional — loaded only when target requests them)
+continuations.sx    depends on: eval (optional)
+callcc.sx           depends on: eval (optional)")))
+
+      (div :class "space-y-3"
+        (h2 :class "text-2xl font-semibold text-stone-800" "Extensions")
+        (p :class "text-stone-600"
+          "Optional bolt-on specifications that extend the core language. Bootstrappers include them only when the target requests them. Code that doesn't use extensions pays zero cost.")
+        (div :class "overflow-x-auto rounded border border-stone-200"
+          (table :class "w-full text-left text-sm"
+            (thead (tr :class "border-b border-stone-200 bg-stone-100"
+              (th :class "px-3 py-2 font-medium text-stone-600" "File")
+              (th :class "px-3 py-2 font-medium text-stone-600" "Role")
+              (th :class "px-3 py-2 font-medium text-stone-600" "Recommended targets")))
+            (tbody
+              (tr :class "border-b border-stone-100"
+                (td :class "px-3 py-2 font-mono text-sm text-violet-700"
+                  (a :href "/specs/continuations" :class "hover:underline"
+                     :sx-get "/specs/continuations" :sx-target "#main-panel" :sx-select "#main-panel"
+                     :sx-swap "outerHTML" :sx-push-url "true"
+                     "continuations.sx"))
+                (td :class "px-3 py-2 text-stone-700" "Delimited continuations — shift/reset")
+                (td :class "px-3 py-2 text-stone-500" "All targets"))
+              (tr :class "border-b border-stone-100"
+                (td :class "px-3 py-2 font-mono text-sm text-violet-700"
+                  (a :href "/specs/callcc" :class "hover:underline"
+                     :sx-get "/specs/callcc" :sx-target "#main-panel" :sx-select "#main-panel"
+                     :sx-swap "outerHTML" :sx-push-url "true"
+                     "callcc.sx"))
+                (td :class "px-3 py-2 text-stone-700" "Full first-class continuations — call/cc")
+                (td :class "px-3 py-2 text-stone-500" "Scheme, Haskell"))))))
 
       (div :class "space-y-3"
         (h2 :class "text-2xl font-semibold text-stone-800" "Self-hosting")

sx/sxc/pages/docs.sx

@@ -331,6 +331,14 @@
               :filename (get item "filename") :href (str "/specs/" (get item "slug"))
               :source (read-spec-file (get item "filename"))))
         browser-spec-items))
+    "extensions" (~spec-overview-content
+      :spec-title "Extensions"
+      :spec-files (map (fn (item)
+        (dict :title (get item "title") :desc (get item "desc")
+              :prose (get item "prose")
+              :filename (get item "filename") :href (str "/specs/" (get item "slug"))
+              :source (read-spec-file (get item "filename"))))
+        extension-spec-items))
     :else (let ((spec (find-spec slug)))
       (if spec
         (~spec-detail-content