CST parser: lossless concrete syntax tree for .sx files

New sx_cst.ml: CstAtom, CstList, CstDict node types with leading/trailing
trivia (whitespace + comments). Two projections:
- cst_to_source/cst_file_to_source: exact source reconstruction
- cst_to_ast: strip trivia → Sx_types.value for evaluation

New parse_all_cst/parse_file_cst in sx_parser.ml: parallel CST parser
alongside existing AST parser. Reuses read_string, read_symbol, try_number.
Trivia collected via collect_trivia (replaces skip_whitespace_and_comments).

Round-trip invariant: cst_file_to_source(parse_all_cst(src)) = src
Verified on 13 synthetic tests + 7 real codebase files (101KB evaluator,
parser, primitives, render, tree-tools, engine, io).

CST→AST equivalence: cst_to_ast matches parse_all output.

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

hosts/ocaml/bin/dune

@@ -5,3 +5,7 @@
 (executable
  (name mcp_tree)
  (libraries sx unix yojson str))
+
+(executable
+ (name test_cst)
+ (libraries sx))

hosts/ocaml/bin/test_cst.ml

@@ -0,0 +1,91 @@
+let () =
+  let test_sources = [
+    "(define foo 42)";
+    ";; comment\n(define bar 1)\n\n;; another\n(define baz 2)\n";
+    "(define my-fn\n  (fn (x)\n    ;; check nil\n    (if (nil? x) 0 x)))";
+    "(list 1 2 3)";
+    "{:key \"value\" :num 42}";
+    "'(a b c)";
+    "`(a ,b ,@c)";
+    "(define x \"hello\\nworld\")";
+    ";; top\n;; multi-line\n(define a 1)\n";
+    "";
+    "  \n  ";
+    "(a)\n(b)\n(c)";
+    "(a ;; inline\n  b)";
+  ] in
+  let pass = ref 0 in
+  let fail = ref 0 in
+  List.iter (fun src ->
+    let cst = Sx_parser.parse_all_cst src in
+    let roundtrip = Sx_cst.cst_file_to_source cst.nodes cst.trailing_trivia in
+    if roundtrip = src then begin
+      incr pass;
+      Printf.printf "PASS: %S\n" (if String.length src > 40 then String.sub src 0 40 ^ "..." else src)
+    end else begin
+      incr fail;
+      Printf.printf "FAIL: %S\n  expected: %S\n  got:      %S\n"
+        (if String.length src > 40 then String.sub src 0 40 ^ "..." else src)
+        src roundtrip
+    end
+  ) test_sources;
+  (* Also test CST→AST matches AST parser *)
+  let ast_tests = [
+    "(define foo 42)";
+    "(list 1 2 3)";
+    "{:key \"value\"}";
+    ";; comment\n(define bar 1)";
+  ] in
+  Printf.printf "\nCST→AST equivalence:\n";
+  List.iter (fun src ->
+    let ast_direct = Sx_parser.parse_all src in
+    let cst = Sx_parser.parse_all_cst src in
+    let ast_via_cst = List.map Sx_cst.cst_to_ast cst.nodes in
+    let s1 = String.concat " " (List.map Sx_types.inspect ast_direct) in
+    let s2 = String.concat " " (List.map Sx_types.inspect ast_via_cst) in
+    if s1 = s2 then begin
+      incr pass;
+      Printf.printf "PASS: %S\n" src
+    end else begin
+      incr fail;
+      Printf.printf "FAIL: %S\n  AST:     %s\n  CST→AST: %s\n" src s1 s2
+    end
+  ) ast_tests;
+  (* Test real .sx files from the codebase *)
+  Printf.printf "\nReal file round-trips:\n";
+  let test_file path =
+    try
+      let src = In_channel.with_open_text path In_channel.input_all in
+      let cst = Sx_parser.parse_all_cst src in
+      let roundtrip = Sx_cst.cst_file_to_source cst.nodes cst.trailing_trivia in
+      if roundtrip = src then begin
+        incr pass;
+        Printf.printf "PASS: %s (%d bytes)\n" path (String.length src)
+      end else begin
+        incr fail;
+        (* Find first difference *)
+        let len = min (String.length src) (String.length roundtrip) in
+        let diff_pos = ref len in
+        for i = 0 to len - 1 do
+          if src.[i] <> roundtrip.[i] && !diff_pos = len then diff_pos := i
+        done;
+        Printf.printf "FAIL: %s (diff at byte %d, src=%d rt=%d)\n" path !diff_pos (String.length src) (String.length roundtrip)
+      end
+    with e ->
+      incr fail;
+      Printf.printf "ERROR: %s — %s\n" path (Printexc.to_string e)
+  in
+  let spec_dir = try Sys.getenv "SX_SPEC_DIR" with Not_found -> "spec" in
+  let project_dir = try Sys.getenv "SX_PROJECT_DIR" with Not_found -> "." in
+  List.iter test_file [
+    spec_dir ^ "/evaluator.sx";
+    spec_dir ^ "/parser.sx";
+    spec_dir ^ "/primitives.sx";
+    spec_dir ^ "/render.sx";
+    project_dir ^ "/lib/tree-tools.sx";
+    project_dir ^ "/web/engine.sx";
+    project_dir ^ "/web/io.sx";
+  ];
+
+  Printf.printf "\n%d/%d passed\n" !pass (!pass + !fail);
+  if !fail > 0 then exit 1