ocaml: phase 4 'fun (a, b) -> body' tuple-param destructuring (+4 tests, 553 total)

parse-fun's collect-params now detects '(IDENT, ...)' as a
tuple-pattern parameter (lookahead at peek-tok-at 1/2 distinguishes
from '(x : T)' and '()' cases that try-consume-param! already
handles). For each tuple param it:

  1. parse-pattern to get the full pattern AST
  2. generate a synthetic __pat_N name as the actual fun parameter
  3. push (synth_name, pattern) onto tuple-binds

After parsing the body, wraps it innermost-first with one
'match __pat_N with PAT -> ...' per tuple-param. The user-visible
result is a (:fun (params...) body) where params are all simple
names but the body destructures.

Also retroactively simplifies Hashtbl.keys/values from
'fun pair -> match pair with (k, _) -> k' to plain
'fun (k, _) -> k', closing the iteration-99 workaround.

  (fun (a, b) -> a + b) (3, 7)              = 10
  List.map (fun (a, b) -> a * b)
           [(1, 2); (3, 4); (5, 6)]         = [2; 12; 30]
  List.map (fun (k, _) -> k)
           [("a", 1); ("b", 2)]              = ["a"; "b"]
  (fun a (b, c) d -> a + b + c + d) 1 (2, 3) 4 = 10

lib/ocaml/test.sh

@@ -1370,6 +1370,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5113)
 (eval "(ocaml-run \"Random.init 7; Random.bool ()\")")
 
+;; ── fun (a, b) -> body — tuple param destructure ──────────────
+(epoch 5120)
+(eval "(ocaml-run \"(fun (a, b) -> a + b) (3, 7)\")")
+(epoch 5121)
+(eval "(ocaml-run \"List.map (fun (a, b) -> a * b) [(1, 2); (3, 4); (5, 6)]\")")
+(epoch 5122)
+(eval "(ocaml-run \"List.map (fun (k, _) -> k) [(\\\"a\\\", 1); (\\\"b\\\", 2)]\")")
+(epoch 5123)
+(eval "(ocaml-run \"(fun a (b, c) d -> a + b + c + d) 1 (2, 3) 4\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2177,6 +2187,12 @@ check 5111 "Random.int x3 seed=42 sum"    '152'
 check 5112 "Random.int 10 seed=1"         '0'
 check 5113 "Random.bool seed=7"           'true'
 
+# ── fun (a, b) -> body tuple param ──────────────────────────────
+check 5120 "fun (a, b) -> a + b"          '10'
+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'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"