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>

shared/sx/ref/parser.sx

@@ -246,6 +246,14 @@
                            (and (>= next-ch "0") (<= next-ch "9")))))
                   (read-number)
 
+                ;; Ellipsis (... as a symbol)
+                (and (= ch ".")
+                     (< (+ pos 2) len-src)
+                     (= (nth source (+ pos 1)) ".")
+                     (= (nth source (+ pos 2)) "."))
+                  (do (set! pos (+ pos 3))
+                      (make-symbol "..."))
+
                 ;; Symbol (must be ident-start char)
                 (ident-start? ch)
                   (read-symbol)