ocaml: phase 1+3 record type declarations (+3 tests, 447 total)

type r = { x : int; mutable y : string } parses to
(:type-def-record NAME PARAMS FIELDS) with FIELDS each (NAME) or
(:mutable NAME). Parser dispatches on { after = to parse field list.
Field-type sources are skipped (HM registration TBD). Runtime no-op
since records already work as dynamic dicts.

plans/ocaml-on-sx.md

@@ -403,6 +403,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 1+3 — record type declarations `type r = { x : int;
+  mutable y : string }` (+3 tests, 447 total). Parser dispatches on
+  `{` after `=` to parse field list (`mutable` keyword tracked).
+  AST: `(:type-def-record NAME PARAMS FIELDS)` with FIELDS each being
+  `(NAME)` or `(:mutable NAME)`. Runtime is no-op (records already
+  work as dynamic dicts). Field-type sources are skipped; HM type
+  registration deferred.
 - 2026-05-08 Phase 5.1 — fizzbuzz.ml baseline (12/12 pass). Classic
   fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif chain,
   String.concat, Int.to_string. Verifies output via String.length.