Fix orchestration.sx parse error, add parser line/col diagnostics

The parser was reporting "Unexpected char: )" with no position info.
Added line number, column, and byte position to all parse errors.

Root cause: bind-sse-swap had one extra close paren that naive paren
counting missed because a "(" exists inside a string literal on L1074
(starts-with? trimmed "("). Parse-aware counting (skipping strings
and comments) correctly identified the imbalance.

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

hosts/ocaml/lib/sx_parser.ml

@@ -95,7 +95,11 @@ let try_number str =
 
 let rec read_value s : value =
   skip_whitespace_and_comments s;
-  if at_end s then raise (Parse_error "Unexpected end of input");
+  if at_end s then begin
+    let line = ref 1 in
+    String.iter (fun c -> if c = '\n' then incr line) s.src;
+    raise (Parse_error (Printf.sprintf "Unexpected end of input at line %d (pos %d)" !line s.pos))
+  end;
   match s.src.[s.pos] with
   | '(' -> read_list s ')'
   | '[' -> read_list s ']'
@@ -139,7 +143,14 @@ let rec read_value s : value =
     begin
       (* Symbol, keyword, number, or boolean *)
       let token = read_symbol s in
-      if token = "" then raise (Parse_error ("Unexpected char: " ^ String.make 1 s.src.[s.pos]));
+      if token = "" then begin
+        let line = ref 1 and col = ref 1 in
+        for i = 0 to s.pos - 1 do
+          if s.src.[i] = '\n' then (incr line; col := 1) else incr col
+        done;
+        raise (Parse_error (Printf.sprintf "Unexpected char: %c at line %d col %d (pos %d)"
+          s.src.[s.pos] !line !col s.pos))
+      end;
       match token with
       | "true" -> Bool true
       | "false" -> Bool false

shared/static/wasm/sx/orchestration.sx

@@ -1084,7 +1084,7 @@
           (with-transition use-transition
             (fn ()
               (swap-html-string target trimmed swap-style)
-              (post-swap target)))))))))
+              (post-swap target))))))))
 
 
 ;; --------------------------------------------------------------------------

web/orchestration.sx

@@ -1084,7 +1084,7 @@
           (with-transition use-transition
             (fn ()
               (swap-html-string target trimmed swap-style)
-              (post-swap target)))))))))
+              (post-swap target))))))))
 
 
 ;; --------------------------------------------------------------------------