ocaml: parser accepts if/match/let/fun as rhs of <- and :=

Previously `a.(i) <- if c then x else y` failed with
"unexpected token keyword if" because parse-binop-rhs called
parse-prefix for the rhs, which doesn't accept if/match/let/fun.

Real OCaml allows full expressions on the rhs of <-/:=. Fix:
special-case prec-1 ops in parse-binop-rhs to call parse-expr-no-seq
instead of parse-prefix. The recursive parse-binop-rhs with
min-prec restored after picks up any further chained <- (since both
ops are right-associative with no higher-prec binops above them).

Manacher baseline updated to use bare `if` on rhs of <-,
removing the parens workaround from iter 235. 607/607 regressions
remain clean.

plans/ocaml-on-sx.md

@@ -407,16 +407,22 @@ _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.
+- 2026-05-10 Phase 5.1 — parser: accept `if/match/let/fun/...` as
+  the rhs of `<-` and `:=`. parse-binop-rhs now special-cases prec-1
+  ops (`<-`, `:=`) to call parse-expr-no-seq for their right operand
+  instead of parse-prefix. Real OCaml accepts `a.(i) <- if c then x
+  else y`, `r := match e with | A -> 1 | B -> 2`, etc. Manacher
+  baseline (iter 235) updated to use bare `if` on rhs of `<-`,
+  matching the workaround note left in its previous Progress entry.
+  607/607 regressions clean. Useful improvement for hot inner-loop
+  code that needs conditional rhs.
 - 2026-05-10 Phase 5.1 — manacher.ml baseline (Manacher's longest
   palindromic substring on "babadaba" = 7). Inserts `#` separators
   to unify odd/even cases (string of length 2n+1), then maintains
   palindrome radii in `p[]` plus a current rightmost-reach pair
   (center, right). Linear time. The full input "babadaba" itself is
   not a palindrome, but "abadaba" (positions 1..7) is — length 7.
-  Note: `p.(i) <- if pm < v then pm else v` requires explicit
-  parens — our parser doesn't accept `if` as the rhs of `<-` at
-  binop level (real OCaml does); workaround until we relax binop
-  RHS parsing. 149 baseline programs total.
+  149 baseline programs total.
 - 2026-05-10 Phase 5.1 — floyd_warshall.ml baseline (all-pairs
   shortest path on 4-node weighted graph, dist 0→3 = 9). Standard
   O(n³) DP: for each intermediate vertex k, relax all (i,j) pairs.