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/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