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/transpiler.sx

@@ -93,6 +93,11 @@
   "*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"