Request primitives, POST support, handler dispatch

OCaml server:
- Accept POST/PUT/PATCH/DELETE for /sx/ paths (was GET-only)
- Parse request body, query string, set per-request context
- Add 16 request primitives: now, state-get/set!, request-form/arg,
  request-json, request-header(s), request-method/body, into, etc.
- URL-encoded body parser for form submissions

Handler dispatch (sx/sx/handlers/dispatch.sx):
- `api` function routes URL paths like (api "click") to handler:ex-click
- `call-handler` checks HTTP method, binds params, evaluates body
- Handlers defined via defhandler in handlers/examples.sx now reachable

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/handlers/dispatch.sx

@@ -0,0 +1,51 @@
+(define
+  api
+  (fn
+    (slug)
+    (let
+      ((handler-key (str "handler:ex-" slug)) (hdef (env-get handler-key)))
+      (if
+        (nil? hdef)
+        (div
+          :class "p-8"
+          (h2 :class "text-rose-600 font-semibold" "Handler not found")
+          (p :class "text-stone-500" (str "No handler: " handler-key)))
+        (call-handler hdef)))))
+
+(define
+  call-handler
+  (fn
+    (hdef)
+    (let
+      ((method (get hdef "method"))
+        (params (get hdef "params"))
+        (body (get hdef "body"))
+        (closure (get hdef "closure")))
+      (let
+        ((req-method (request-method))
+          (handler-method (upper (or method "get"))))
+        (if
+          (and
+            (not (= handler-method "GET"))
+            (not (= req-method handler-method)))
+          (div
+            :class "p-4 text-rose-600"
+            (str
+              "Method not allowed: expected "
+              handler-method
+              " got "
+              req-method))
+          (eval-handler-body params body closure))))))
+
+(define
+  eval-handler-body
+  (fn
+    (params body closure)
+    (if
+      (or (nil? params) (empty? params))
+      (eval-expr body closure)
+      (let
+        ((bindings (map (fn (p) (let ((name (if (symbol? p) (symbol-name p) (str p)))) (if (= name "&key") nil (if (= name "&rest") nil (let ((val (or (request-arg name) (request-form name)))) (list (make-symbol name) (or val nil))))))) (filter (fn (p) (let ((name (if (symbol? p) (symbol-name p) (str p)))) (and (not (= name "&key")) (not (= name "&rest"))))) params))))
+        (let
+          ((let-form (list (quote let) (filter (fn (b) (not (nil? b))) bindings) body)))
+          (eval-expr let-form closure))))))