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/parser.py

@@ -170,6 +170,11 @@ class Tokenizer:
                     return float(num_str)
                 return int(num_str)
 
+        # Ellipsis (... as a symbol, used in spec declarations)
+        if char == "." and self.text[self.pos:self.pos + 3] == "...":
+            self._advance(3)
+            return Symbol("...")
+
         # Symbol
         m = self.SYMBOL.match(self.text, self.pos)
         if m: