ocaml: phase 4 'let PATTERN = expr in body' tuple destructuring (+3 tests, 541 total)

When 'let' is followed by '(', parse-let now reads a full pattern
(via the existing parse-pattern used by match), expects '=', then
'in', and desugars to:

  let PATTERN = EXPR in BODY  =>  match EXPR with PATTERN -> BODY

This reuses the entire pattern-matching machinery, so any pattern
the match parser accepts works here too — paren-tuples, nested
tuples, cons patterns, list patterns. No 'rec' allowed for pattern
bindings (real OCaml's restriction).

  let (a, b) = (1, 2) in a + b              = 3
  let (a, b, c) = (10, 20, 30) in a + b + c = 60
  let pair = (5, 7) in
  let (x, y) = pair in x * y                = 35

Also retroactively cleaned up Printf's iter-97 width-pos packing
hack ('width * 1000000 + spec_pos') — it's now
'let (width, spec_pos) = parse_width_loop after_flags in ...' like
real OCaml.

plans/ocaml-on-sx.md

@@ -407,6 +407,14 @@ _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.
+- 2026-05-09 Phase 4 — `let PATTERN = expr in body` tuple
+  destructuring (+3 tests, 541 total). When `let` is followed by `(`,
+  parse-let now reads a full pattern, expects `=`, then `in`, and
+  desugars to `(:match expr ((:case PATTERN body)))`. Reuses the
+  pattern parser used by match. `let (a, b, c) = (10, 20, 30) in
+  a+b+c` → 60. Also retroactively cleans up the Printf width-pos
+  packing hack from iteration 97 — it's now `let (width, spec_pos)
+  = parse_width_loop after_flags in ...` like real OCaml.
 - 2026-05-09 Phase 6 — Printf width specifiers `%5d` / `%-5d` /
   `%05d` / `%4s` etc. (+5 tests, 538 total). Walker now parses
   optional `-` (left-align) and `0` (zero-pad) flags after `%`, then