Decouple core evaluator from web platform, extract libraries

The core evaluator (spec/evaluator.sx) is now the irreducible computational
core with zero web, rendering, or type-system knowledge. 2531 → 2313 lines.

- Add extensible special form registry (*custom-special-forms* + register-special-form!)
- Add render dispatch hooks (*render-check* / *render-fn*) replacing hardcoded render-active?/is-render-expr?/render-expr
- Extract freeze scopes → spec/freeze.sx (library, not core)
- Extract content addressing → spec/content.sx (library, not core)
- Move sf-deftype/sf-defeffect → spec/types.sx (self-registering)
- Move sf-defstyle → web/forms.sx (self-registering with all web forms)
- Move web tests (defpage, streaming) → web/tests/test-forms.sx
- Add is-else-clause? helper (replaces 5 inline patterns)
- Make escape-html/escape-attr library functions in render.sx (pure SX, not platform-provided)
- Add foundations plan: Step 3.5 (data representations), Step 3.7 (verified components), OCaml for Step 4d
- Update all three bootstrappers (JS 957/957, Python 744/744, OCaml 952/952)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/javascript/platform.py

@@ -13,7 +13,14 @@ from shared.sx.types import Symbol
 
 
 def extract_defines(source: str) -> list[tuple[str, list]]:
-    """Parse .sx source, return list of (name, define-expr) for top-level defines."""
+    """Parse .sx source, return list of (name, expr) for top-level forms.
+
+    Extracts (define name ...) forms with their name, plus selected
+    non-define top-level expressions (e.g. register-special-form! calls)
+    with a synthetic name for the comment.
+    """
+    # Top-level calls that should be transpiled (not special forms)
+    _TOPLEVEL_CALLS = {"register-special-form!"}
     exprs = parse_all(source)
     defines = []
     for expr in exprs:
@@ -21,6 +28,10 @@ def extract_defines(source: str) -> list[tuple[str, list]]:
             if expr[0].name == "define":
                 name = expr[1].name if isinstance(expr[1], Symbol) else str(expr[1])
                 defines.append((name, expr))
+            elif expr[0].name in _TOPLEVEL_CALLS:
+                # Top-level call expression (e.g. register-special-form!)
+                call_name = expr[0].name
+                defines.append((f"({call_name} ...)", expr))
     return defines
 
 ADAPTER_FILES = {
@@ -283,9 +294,11 @@ ASYNC_IO_JS = '''
       if (hname === "map-indexed") return asyncRenderMapIndexed(expr, env, ns);
       if (hname === "for-each") return asyncRenderMap(expr, env, ns);
 
-      // define/defcomp/defmacro — eval for side effects
+      // define/defcomp/defmacro and custom special forms — eval for side effects
       if (hname === "define" || hname === "defcomp" || hname === "defmacro" ||
-          hname === "defstyle" || hname === "defhandler") {
+          hname === "defstyle" || hname === "defhandler" ||
+          hname === "deftype" || hname === "defeffect" ||
+          (typeof _customSpecialForms !== "undefined" && _customSpecialForms[hname])) {
         trampoline(evalExpr(expr, env));
         return null;
       }
@@ -1412,10 +1425,7 @@ PLATFORM_JS_POST = '''
   var dict_fn = PRIMITIVES["dict"];
 
   // HTML rendering helpers
-  function escapeHtml(s) {
-    return String(s).replace(/&/g,"&amp;").replace(/</g,"&lt;").replace(/>/g,"&gt;").replace(/"/g,"&quot;");
-  }
-  function escapeAttr(s) { return escapeHtml(s); }
+  // escape-html and escape-attr are now library functions defined in render.sx
   function rawHtmlContent(r) { return r.html; }
   function makeRawHtml(s) { return { _raw: true, html: s }; }
   function sxExprSource(x) { return x && x.source ? x.source : String(x); }
@@ -1429,7 +1439,8 @@ PLATFORM_JS_POST = '''
 
   function isDefinitionForm(name) {
     return name === "define" || name === "defcomp" || name === "defmacro" ||
-           name === "defstyle" || name === "defhandler";
+           name === "defstyle" || name === "defhandler" ||
+           name === "deftype" || name === "defeffect";
   }
 
   function indexOf_(s, ch) {
@@ -1703,6 +1714,11 @@ PLATFORM_DOM_JS = """
   _renderExprFn = function(expr, env) { return renderToDom(expr, env, null); };
   _renderMode = true;  // Browser always evaluates in render context.
 
+  // Wire CEK render hooks — evaluator checks _renderCheck/_renderFn instead of
+  // the old renderActiveP()/isRenderExpr()/renderExpr() triple.
+  _renderCheck = function(expr, env) { return isRenderExpr(expr); };
+  _renderFn = function(expr, env) { return renderToDom(expr, env, null); };
+
   var SVG_NS = "http://www.w3.org/2000/svg";
   var MATH_NS = "http://www.w3.org/1998/Math/MathML";