Document why Std_ast exists

Add docstring explaining that the standard parser is used to verify
formatting preserves semantics: two programs are equivalent if the
compiler parses them identically.

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

Author: hhugo

lib/Std_ast.mli

@@ -9,7 +9,18 @@
 (*                                                                        *)
 (**************************************************************************)
 
-(** Interface over the AST defined in vendor/ocaml-4.13 *)
+(** Standard OCaml parser AST (vendor/parser-standard).
+
+    OCamlformat uses an extended parser for formatting that supports
+    additional syntax constructs (field punning, begin..end nodes, labeled
+    tuples, etc.). To verify that formatting preserves semantics,
+    ocamlformat re-parses both the original and formatted source with the
+    standard OCaml parser and compares the resulting ASTs. If they differ,
+    formatting changed the meaning of the program — which is a bug.
+
+    The standard parser is used for this comparison because it represents
+    what the OCaml compiler sees: two programs are equivalent if the
+    compiler parses them identically. *)
 
 open Ocamlformat_parser_standard