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/content.sx

@@ -0,0 +1,48 @@
+;; ==========================================================================
+;; content.sx — Content-addressed computation
+;;
+;; Hash frozen SX to a content identifier. Store and retrieve by CID.
+;; The content IS the address — same SX always produces the same CID.
+;;
+;; This is a library built on top of freeze.sx. It is NOT part of the
+;; core evaluator. Load order: evaluator.sx → freeze.sx → content.sx
+;;
+;; Uses an in-memory content store. Applications can persist to
+;; localStorage or IPFS by providing their own store backend.
+;; ==========================================================================
+
+(define content-store (dict))
+
+(define content-hash :effects []
+  (fn (sx-text)
+    ;; djb2 hash → hex string. Simple, deterministic, fast.
+    ;; Real deployment would use SHA-256 / multihash.
+    (let ((hash 5381))
+      (for-each (fn (i)
+        (set! hash (mod (+ (* hash 33) (char-code-at sx-text i)) 4294967296)))
+        (range 0 (len sx-text)))
+      (to-hex hash))))
+
+(define content-put :effects [mutation]
+  (fn (sx-text)
+    (let ((cid (content-hash sx-text)))
+      (dict-set! content-store cid sx-text)
+      cid)))
+
+(define content-get :effects []
+  (fn (cid)
+    (get content-store cid)))
+
+;; Freeze a scope → store → return CID
+(define freeze-to-cid :effects [mutation]
+  (fn (scope-name)
+    (let ((sx-text (freeze-to-sx scope-name)))
+      (content-put sx-text))))
+
+;; Thaw from CID → look up → restore
+(define thaw-from-cid :effects [mutation]
+  (fn (cid)
+    (let ((sx-text (content-get cid)))
+      (when sx-text
+        (thaw-from-sx sx-text)
+        true))))