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/bootstrap.py

@@ -179,6 +179,11 @@ class PyEmitter:
             "*batch-depth*": "_batch_depth",
             "*batch-queue*": "_batch_queue",
             "*store-registry*": "_store_registry",
+            "*custom-special-forms*": "_custom_special_forms",
+            "*render-check*": "_render_check",
+            "*render-fn*": "_render_fn",
+            "register-special-form!": "register_special_form_b",
+            "is-else-clause?": "is_else_clause_p",
             "def-store": "def_store",
             "use-store": "use_store",
             "clear-stores": "clear_stores",