ocaml: phase 1 type annotations on let / (e : T) (+4 tests, 473 total)

let NAME [PARAMS] : T = expr and (expr : T) parse and skip the type
source. Runtime no-op since SX is dynamic. Works in inline let,
top-level let, and parenthesised expressions:

  let x : int = 5 ;; x + 1                        -> 6
  let f (x : int) : int = x + 1 in f 41           -> 42
  (5 : int)                                       -> 5
  ((1 + 2) : int) * 3                             -> 9

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — type annotations on let-bindings and parens
+  expressions (+4 tests, 473 total). `let NAME [PARAMS] : T = expr`
+  and `(expr : T)` parse and skip the type source. Runtime no-op
+  (dynamic). Works in inline let, top-level let, and parenthesised
+  expressions: `let f (x : int) : int = x + 1 in f 41`.
 - 2026-05-08 Phase 1+5.1 — type aliases + poly_stack baseline (+3
   tests, 469 total + 19 baseline). Parser dispatch on the post-`=`
   token: `|` or `Ctor` → sum, `{` → record, otherwise → alias (skip