ocaml: phase 4 'let f (a, b) = body' tuple-param on inner-let (+3 tests, 556 total)

Mirrors iteration 101's parse-fun change inside parse-let's
parse-one!:

  - same '(IDENT, ...)' detection on collect-params
  - same __pat_N synth name for the function param
  - same innermost-first match-wrapping

Difference: for inner-let the wrapping is applied to the rhs of the
let-binding (which is the function value), not directly to a fun
body.

  let f (a, b) = a + b in f (3, 7)            = 10
  let g x (a, b) = x + a + b in g 1 (2, 3)    = 6
  let h (a, b) (c, d) = a * b + c * d
    in h (1, 2) (3, 4)                        = 14

lib/ocaml/test.sh

@@ -1380,6 +1380,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5123)
 (eval "(ocaml-run \"(fun a (b, c) d -> a + b + c + d) 1 (2, 3) 4\")")
 
+;; ── let f (a, b) = body — tuple param on inner-let ────────────
+(epoch 5130)
+(eval "(ocaml-run \"let f (a, b) = a + b in f (3, 7)\")")
+(epoch 5131)
+(eval "(ocaml-run \"let g x (a, b) = x + a + b in g 1 (2, 3)\")")
+(epoch 5132)
+(eval "(ocaml-run \"let h (a, b) (c, d) = a * b + c * d in h (1, 2) (3, 4)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2193,6 +2201,11 @@ check 5121 "List.map fun (a, b)"          '(2 12 30)'
 check 5122 "List.map fun (k, _)"          '("a" "b")'
 check 5123 "fun a (b, c) d mixed"         '10'
 
+# ── let f (a, b) = body — let with tuple param ──────────────────
+check 5130 "let f (a, b) = a + b"         '10'
+check 5131 "let g x (a, b) mixed"         '6'
+check 5132 "let h (a, b) (c, d) curried"  '14'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"