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/python/tests/run_tests.py

@@ -248,9 +248,26 @@ env["macro-closure"] = lambda m: m.closure
 env["symbol-name"] = lambda s: s.name if isinstance(s, Symbol) else str(s)
 env["keyword-name"] = lambda k: k.name if isinstance(k, Keyword) else str(k)
 env["sx-serialize"] = sx_ref.sx_serialize if hasattr(sx_ref, "sx_serialize") else lambda x: str(x)
-env["is-render-expr?"] = lambda expr: False
-env["render-active?"] = lambda: False
-env["render-expr"] = lambda expr, env: NIL
+
+# Render dispatch globals — evaluator checks *render-check* and *render-fn*
+env["*render-check*"] = NIL
+env["*render-fn*"] = NIL
+
+# Custom special forms registry — modules register forms at load time
+env["*custom-special-forms*"] = {}
+def _register_special_form(name, handler):
+    env["*custom-special-forms*"][name] = handler
+    return NIL
+env["register-special-form!"] = _register_special_form
+
+# is-else-clause? — check if a cond/case test is an else marker
+def _is_else_clause(test):
+    if isinstance(test, Keyword) and test.name == "else":
+        return True
+    if isinstance(test, Symbol) and test.name in ("else", ":else"):
+        return True
+    return False
+env["is-else-clause?"] = _is_else_clause
 
 # Strict mode stubs (not yet bootstrapped to Python — no-ops for now)
 env["set-strict!"] = lambda val: NIL