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>

spec/render.sx

@@ -146,11 +146,7 @@
       (let ((clause (first clauses))
             (test   (first clause))
             (body   (nth clause 1)))
-        (if (or (and (= (type-of test) "symbol")
-                     (or (= (symbol-name test) "else")
-                         (= (symbol-name test) ":else")))
-                (and (= (type-of test) "keyword")
-                     (= (keyword-name test) "else")))
+        (if (is-else-clause? test)
           body
           (if (trampoline (eval-expr test env))
             body
@@ -162,10 +158,7 @@
       nil
       (let ((test (first clauses))
             (body (nth clauses 1)))
-        (if (or (and (= (type-of test) "keyword") (= (keyword-name test) "else"))
-                (and (= (type-of test) "symbol")
-                     (or (= (symbol-name test) "else")
-                         (= (symbol-name test) ":else"))))
+        (if (is-else-clause? test)
           body
           (if (trampoline (eval-expr test env))
             body
@@ -250,13 +243,28 @@
       (keys spread-dict))))
 
 
+;; --------------------------------------------------------------------------
+;; HTML escaping — library functions (pure text processing)
+;; --------------------------------------------------------------------------
+
+(define escape-html
+  (fn (s)
+    (-> (str s)
+      (replace "&" "&")
+      (replace "<" "&lt;")
+      (replace ">" "&gt;")
+      (replace "\"" "&quot;"))))
+
+(define escape-attr
+  (fn (s)
+    (escape-html s)))
+
+
 ;; --------------------------------------------------------------------------
 ;; Platform interface (shared across adapters)
 ;; --------------------------------------------------------------------------
 ;;
-;; HTML/attribute escaping (used by HTML and SX wire adapters):
-;;   (escape-html s)           → HTML-escaped string
-;;   (escape-attr s)           → attribute-value-escaped string
+;; Raw HTML (marker type for unescaped content):
 ;;   (raw-html-content r)      → unwrap RawHTML marker to string
 ;;
 ;; Spread (render-time attribute injection):