Make continuations an optional extension, add special-forms.sx, ellipsis parsing

- Both bootstrappers (JS + Python) now gate shift/reset behind --extensions
  continuations flag. Without it, using reset/shift errors at runtime.
- JS bootstrapper: extracted Continuation/ShiftSignal types, sfReset/sfShift,
  continuation? primitive, and typeOf handling into CONTINUATIONS_JS constant.
  Extension wraps evalList, aserSpecial, and typeOf post-transpilation.
- Python bootstrapper: added special-forms.sx validation cross-check against
  eval.sx dispatch, warns on mismatches.
- Added shared/sx/ref/special-forms.sx: 36 declarative form specs with syntax,
  docs, tail-position, and examples. Used by bootstrappers for validation.
- Added ellipsis (...) support to both parser.py and parser.sx spec.
- Updated continuations essay to reflect optional extension architecture.
- Updated specs page and nav with special-forms.sx entry.

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

sx/sx/nav-data.sx

@@ -86,6 +86,7 @@
   (dict :label "Parser" :href "/specs/parser")
   (dict :label "Evaluator" :href "/specs/evaluator")
   (dict :label "Primitives" :href "/specs/primitives")
+  (dict :label "Special Forms" :href "/specs/special-forms")
   (dict :label "Renderer" :href "/specs/renderer")
   (dict :label "Adapters" :href "/specs/adapters")
   (dict :label "DOM Adapter" :href "/specs/adapter-dom")
@@ -119,6 +120,9 @@
   (dict :slug "primitives" :filename "primitives.sx"  :title "Primitives"
         :desc "All built-in pure functions and their signatures."
         :prose "Primitives are the built-in functions available in every SX environment. Each entry declares a name, parameter signature, and semantics. Bootstrap compilers implement these natively per target (JavaScript, Python, etc.). The registry covers arithmetic, comparison, string manipulation, list operations, dict operations, type predicates, and control flow helpers. All primitives are pure — they take values and return values with no side effects. Platform-specific operations (DOM access, HTTP, file I/O) are provided separately via platform bridge functions, not primitives.")
+  (dict :slug "special-forms" :filename "special-forms.sx" :title "Special Forms"
+        :desc "All special forms — syntactic constructs with custom evaluation rules."
+        :prose "Special forms are the syntactic constructs whose arguments are NOT evaluated before dispatch. Each form has its own evaluation rules — unlike primitives, which receive pre-evaluated values. Together with primitives, special forms define the complete language surface. The registry covers control flow (if, when, cond, case, and, or), binding (let, letrec, define, set!), functions (lambda, defcomp, defmacro), sequencing (begin, do, thread-first), quoting (quote, quasiquote), continuations (reset, shift), guards (dynamic-wind), higher-order forms (map, filter, reduce), and domain-specific definitions (defstyle, defhandler, defpage, defquery, defaction).")
   (dict :slug "renderer"   :filename "render.sx"      :title "Renderer"
         :desc "Shared rendering registries and utilities used by all adapters."
         :prose "The renderer defines what is renderable and how arguments are parsed, but not the output format. It maintains registries of known HTML tags, SVG tags, void elements, and boolean attributes. It specifies how keyword arguments on elements become HTML attributes, how children are collected, and how special attributes (class, style, data-*) are handled. All three adapters (DOM, HTML, SX wire) share these definitions so they agree on what constitutes valid markup. The renderer also defines the StyleValue type used by the CSSX on-demand CSS system.")))

sx/sx/specs.sx

@@ -47,6 +47,13 @@
                      :sx-swap "outerHTML" :sx-push-url "true"
                      "primitives.sx"))
                 (td :class "px-3 py-2 text-stone-700" "All built-in pure functions and their signatures"))
+              (tr :class "border-b border-stone-100"
+                (td :class "px-3 py-2 font-mono text-sm text-violet-700"
+                  (a :href "/specs/special-forms" :class "hover:underline"
+                     :sx-get "/specs/special-forms" :sx-target "#main-panel" :sx-select "#main-panel"
+                     :sx-swap "outerHTML" :sx-push-url "true"
+                     "special-forms.sx"))
+                (td :class "px-3 py-2 text-stone-700" "All special forms — syntactic constructs with custom evaluation rules"))
               (tr :class "border-b border-stone-100"
                 (td :class "px-3 py-2 font-mono text-sm text-violet-700"
                   (a :href "/specs/renderer" :class "hover:underline"
@@ -157,7 +164,8 @@
           (pre :class "text-sm leading-relaxed whitespace-pre-wrap break-words font-mono text-stone-700"
 "parser.sx          (standalone — no dependencies)
 primitives.sx       (standalone — declarative registry)
-eval.sx             depends on: parser, primitives
+special-forms.sx    (standalone — declarative registry)
+eval.sx             depends on: parser, primitives, special-forms
 render.sx           (standalone — shared registries)
 
 adapter-dom.sx      depends on: render, eval