Fix POST body reading + handler param binding for POST requests

Two fixes in the HTTP server:

1. Read full POST body: the single 8192-byte read() could miss the body
   if it arrived in a separate TCP segment. Now parses Content-Length
   and reads remaining bytes in a loop.

2. Handler param binding: for POST/PUT/PATCH, check request-form before
   request-arg. The old (or (request-arg n) (request-form n)) pattern
   short-circuited on request-arg's "" default (truthy in SX), never
   reaching request-form.

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

hosts/ocaml/bin/sx_server.ml

@@ -2994,15 +2994,19 @@ let http_mode port =
                   | Symbol s when s <> "&key" && s <> "&rest" -> Some s
                   | String s when s <> "&key" && s <> "&rest" -> Some s
                   | _ -> None) params in
-                (* Bind handler params in env before aser *)
+                (* Bind handler params in env before aser.
+                   Try request-form first for POST (request-arg returns "" as default,
+                   which is truthy in SX, preventing or-fallback to request-form). *)
+                let is_post = req_method = "POST" || req_method = "PUT" || req_method = "PATCH" in
                 List.iter (fun n ->
-                  let v = try Sx_ref.eval_expr
-                    (List [Symbol "or"; List [Symbol "request-arg"; String n];
-                      List [Symbol "request-form"; String n]]) (Env env)
+                  let v = try
+                    let form_val = Sx_ref.eval_expr (List [Symbol "request-form"; String n]) (Env env) in
+                    let arg_val = Sx_ref.eval_expr (List [Symbol "request-arg"; String n]) (Env env) in
+                    if is_post then (if form_val <> String "" then form_val else arg_val)
+                    else (if arg_val <> Nil then arg_val else form_val)
                     with _ -> Nil in
                   ignore (env_bind env n v)
                 ) param_names;
-                _pending_response_status := 200;
                 let aser_call = List [Symbol "aser"; List [Symbol "quote"; body]; Env env] in
                 let body_str = match Sx_ref.eval_expr aser_call (Env env) with
                   | String s | SxExpr s -> s | v -> Sx_types.inspect v in
@@ -3091,10 +3095,47 @@ let http_mode port =
       (* Read request — non-blocking: set a short timeout *)
       Unix.setsockopt_float client Unix.SO_RCVTIMEO 5.0;
       Unix.setsockopt_float client Unix.SO_SNDTIMEO 10.0;
-      let buf = Bytes.create 8192 in
-      let n = try Unix.read client buf 0 8192 with _ -> 0 in
+      let buf = Buffer.create 8192 in
+      let tmp = Bytes.create 8192 in
+      let n = try Unix.read client tmp 0 8192 with _ -> 0 in
+      Buffer.add_subbytes buf tmp 0 n;
+      (* For POST: ensure full body is read based on Content-Length *)
       if n > 0 then begin
-        let data = Bytes.sub_string buf 0 n in
+        let initial = Buffer.contents buf in
+        let header_end =
+          let rec find s i = if i + 4 > String.length s then -1
+            else if String.sub s i 4 = "\r\n\r\n" then i + 4
+            else find s (i + 1) in find initial 0 in
+        if header_end > 0 then begin
+          (* Parse Content-Length from headers *)
+          let headers_str = String.lowercase_ascii (String.sub initial 0 header_end) in
+          let content_length =
+            let rec find_cl s i =
+              if i + 16 > String.length s then 0
+              else if String.sub s i 16 = "content-length: " then
+                let start = i + 16 in
+                let end_ = try String.index_from s start '\r' with Not_found ->
+                  try String.index_from s start '\n' with Not_found -> String.length s in
+                (try int_of_string (String.trim (String.sub s start (end_ - start))) with _ -> 0)
+              else find_cl s (i + 1) in find_cl headers_str 0 in
+          let body_so_far = String.length initial - header_end in
+          let remaining = content_length - body_so_far in
+          if remaining > 0 then begin
+            let body_buf = Bytes.create remaining in
+            let read_so_far = ref 0 in
+            while !read_so_far < remaining do
+              let r = try Unix.read client body_buf !read_so_far (remaining - !read_so_far)
+                      with _ -> 0 in
+              if r = 0 then read_so_far := remaining  (* EOF *)
+              else read_so_far := !read_so_far + r
+            done;
+            Buffer.add_subbytes buf body_buf 0 !read_so_far
+          end
+        end
+      end;
+      let n = Buffer.length buf in
+      if n > 0 then begin
+        let data = Buffer.contents buf in
         let is_ajax = is_sx_request data in
         if is_ajax then Printf.eprintf "[sx-http] AJAX request detected\n%!";
         let handled =