ocaml: phase 4 top-level 'let f (a, b) = body' tuple-param decl (+3 tests, 559 total)

parse-decl-let lives in the outer ocaml-parse-program scope and does
not have access to parse-pattern (which is local to ocaml-parse).
Source-slicing approach instead:

  1. detect '(IDENT, ...)' in collect-params
  2. scan tokens to the matching ')' (tracking nested parens)
  3. slice the pattern source string from src
  4. push (synth_name, pat_src) onto tuple-srcs

Then after collecting params, the rhs source string gets wrapped with
'match SN with PAT_SRC -> (RHS_SRC)' for each tuple-param,
innermost-first, and the final string is fed through ocaml-parse.

End result is the same AST shape as the iteration-102 inner-let
case: a function whose body destructures a synthetic name.

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

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _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 — top-level `let f (a, b) = body` tuple-param
+  decl (+3 tests, 559 total). parse-decl-let (which lives outside
+  the ocaml-parse scope and lacks parse-pattern access) uses a
+  source-slicing approach: detect `(IDENT, ...)`, scan tokens to
+  matching `)`, slice the pattern source string, store as
+  (synth_name, pat_src). After collecting params, wrap the rhs
+  source string with `match SN with PAT_SRC -> (RHS_SRC)` for each
+  tuple-param, innermost first, then ocaml-parse the wrapped
+  string. End result is the same shape as the inner-let case: a
+  function whose body destructures a synthetic name.
 - 2026-05-09 Phase 4 — `let f (a, b) = body in body2` tuple-param on
   inner-let bindings (+3 tests, 556 total). Mirrors iteration 101's
   parse-fun change inside parse-let's parse-one!: same `(IDENT, ...)`