Preserve ;; comments through MCP tree edit round-trips

Parser gains Comment(string) AST variant and ~comments:true mode that
captures top-level ;; lines instead of discarding them. All MCP edit
tools (replace_node, insert_child, delete_node, wrap_node, rename_symbol,
replace_by_pattern, insert_near, rename_across, pretty_print, write_file)
now preserve comments: separate before tree-tools operate (so index paths
stay correct), re-interleave after editing, emit in pretty_print_file.

Default parse path (evaluator, runtime, compiler) is unchanged — comments
are still stripped unless explicitly requested.

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

hosts/ocaml/lib/sx_parser.ml

@@ -30,6 +30,23 @@ let skip_whitespace_and_comments s =
     | _ -> ()
   in go ()
 
+(* Skip whitespace only — leaves comments for capture *)
+let skip_whitespace s =
+  let rec go () =
+    if at_end s then ()
+    else match s.src.[s.pos] with
+    | ' ' | '\t' | '\n' | '\r' -> advance s; go ()
+    | _ -> ()
+  in go ()
+
+(* Read a comment line starting at ';', returns text after ";;" (or ";") *)
+let read_comment s =
+  let start = s.pos in
+  while s.pos < s.len && s.src.[s.pos] <> '\n' do advance s done;
+  let text = String.sub s.src start (s.pos - start) in
+  if s.pos < s.len then advance s;
+  text
+
 (* Character classification — matches spec/parser.sx ident-start/ident-char.
    ident-start: a-z A-Z _ ~ * + - > < = / ! ? &
    ident-char:  ident-start plus 0-9 . : / # , *)
@@ -202,23 +219,48 @@ and read_dict s =
   in go ()
 
 
-(** Parse a string into a list of SX values. *)
-let parse_all src =
+(** Parse a string into a list of SX values.
+    When [~comments:true], top-level ;; comments are preserved as [Comment]
+    nodes in the result list.  Default is [false] (strip comments). *)
+let parse_all ?(comments=false) src =
   let s = make_state src in
   let results = ref [] in
   let rec go () =
-    skip_whitespace_and_comments s;
-    if at_end s then List.rev !results
-    else begin
-      results := read_value s :: !results;
-      go ()
+    if comments then begin
+      skip_whitespace s;
+      if at_end s then List.rev !results
+      else if s.src.[s.pos] = ';' then begin
+        (* Collect consecutive comment lines into one Comment node *)
+        let lines = ref [] in
+        let rec collect () =
+          skip_whitespace s;
+          if not (at_end s) && s.src.[s.pos] = ';' then begin
+            lines := read_comment s :: !lines;
+            collect ()
+          end
+        in
+        collect ();
+        let text = String.concat "\n" (List.rev !lines) in
+        results := Comment text :: !results;
+        go ()
+      end else begin
+        results := read_value s :: !results;
+        go ()
+      end
+    end else begin
+      skip_whitespace_and_comments s;
+      if at_end s then List.rev !results
+      else begin
+        results := read_value s :: !results;
+        go ()
+      end
     end
   in go ()
 
 (** Parse a file into a list of SX values. *)
-let parse_file path =
+let parse_file ?(comments=false) path =
   let ic = open_in path in
   let n = in_channel_length ic in
   let src = really_input_string ic n in
   close_in ic;
-  parse_all src
+  parse_all ~comments src