ocaml: phase 5.1 baseline OCaml programs (5/5 pass) + lookahead boundary

lib/ocaml/baseline/{factorial,list_ops,option_match,module_use,sum_squares}.ml
exercised through ocaml-run-program (file-read F). lib/ocaml/baseline/
run.sh runs them and compares against expected.json — all 5 pass.

To make module_use.ml (with nested let-in) parse, parser's
skip-let-rhs-boundary! now uses has-matching-in? lookahead: a let at
depth 0 in a let-decl rhs opens a nested block IFF a matching in
exists before any decl-keyword. Without that in, the let is a new
top-level decl (preserves test 274 'let x = 1 let y = 2').

This is the first piece of Phase 5.1 'vendor a slice of OCaml
testsuite' — handcrafted fixtures for now, real testsuite TBD.

plans/ocaml-on-sx.md

@@ -214,10 +214,12 @@ SX CEK evaluator (both JS and OCaml hosts)
       eval-core, phase2-refs, phase2-loops, phase2-function, phase2-exn,
       phase3-adt, phase4-modules, phase5-hm, phase6-stdlib, let-and,
       phase1-params, misc), and emits `scoreboard.json` + `scoreboard.md`.
-- [ ] Vendor a slice of the OCaml testsuite at `lib/ocaml/baseline/`
-      and feed it through `ocaml-run-program`, scoring per-file
-      conformance. _(Pending — needs more stdlib coverage and ADT type
-      decls to make most testsuite files runnable.)_
+- [~] Baseline OCaml programs at `lib/ocaml/baseline/` exercised through
+      `ocaml-run-program`. Currently 5/5: factorial.ml (recursion),
+      list_ops.ml (List.map + fold_left), option_match.ml (option +
+      pattern match), module_use.ml (module + ref + closure +
+      sequenced calls), sum_squares.ml (for-loop + ref). Real OCaml
+      testsuite vendoring is the next step.
 
 ### Phase 5 — Hindley-Milner type inference
 
@@ -370,6 +372,15 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5.1 — `lib/ocaml/baseline/` with five sample OCaml
+  programs (.ml files), driven by `lib/ocaml/baseline/run.sh` through
+  `ocaml-run-program (file-read F)`. All 5/5 pass: factorial,
+  list_ops, option_match, module_use (module + ref + closure +
+  sequenced calls), sum_squares (for-loop). To make module_use parse,
+  parser's `skip-let-rhs-boundary!` now lookaheads for a matching `in`
+  before any decl-keyword — distinguishes nested let-in from a new
+  top-level decl. Test 274 (`let x = 1 let y = 2`) still works because
+  its body has no inner `in`.
 - 2026-05-08 Phase 5 — HM with user `type` declarations (+6 tests, 363
   total). `ocaml-hm-ctors` is now a mutable list cell; user type-defs
   register their constructors via `ocaml-hm-register-type-def!`. New