ocaml: && / || short-circuit fix + bfs_grid.ml baseline (5x5 grid, dist 8)

Before: `:op` handler always evaluated both operands before dispatching
to ocaml-eval-op. For pure binops that's fine, but `&&` / `||` MUST
short-circuit:

  if nr >= 0 && grid.(nr).(nc) = 0 then ...

When nr = -1, real OCaml never evaluates `grid.(-1)`. Our evaluator
did, and crashed with "nth: list/string and number".

Fix: special-case `&&` and `||` in :op dispatch, mirroring the same
pattern already used for `:=` and `<-`. Evaluate lhs, branch on it,
and only evaluate rhs when needed.

Latent since baseline 1 — earlier programs never triggered it because
the rhs was unconditionally safe.

bfs_grid.ml: shortest path through a 5x5 grid with walls. Standard
BFS using Queue.{push,pop,is_empty} + Array.init for the 2D distance
matrix. Path 0,0 -> ... -> 4,4 has length 8. 155 baseline programs
total.

lib/ocaml/baseline/bfs_grid.ml

@@ -0,0 +1,42 @@
+let h = 5
+let w = 5
+
+let grid = [|
+  [| 0; 0; 1; 0; 0 |];
+  [| 1; 0; 1; 0; 1 |];
+  [| 0; 0; 0; 0; 0 |];
+  [| 0; 1; 1; 1; 0 |];
+  [| 0; 0; 0; 0; 0 |]
+|]
+
+let step dist q r c nr nc =
+  if nr >= 0 && nr < h && nc >= 0 && nc < w
+     && grid.(nr).(nc) = 0 && dist.(nr).(nc) = -1 then begin
+    dist.(nr).(nc) <- dist.(r).(c) + 1;
+    Queue.push (nr * 10 + nc) q
+  end
+
+let bfs sr sc tr tc =
+  let dist = Array.init h (fun _ -> Array.make w (-1)) in
+  let q = Queue.create () in
+  dist.(sr).(sc) <- 0;
+  Queue.push (sr * 10 + sc) q;
+  let go = ref true in
+  while !go do
+    if Queue.is_empty q then go := false
+    else if dist.(tr).(tc) <> -1 then go := false
+    else begin
+      let rc = Queue.pop q in
+      let r = rc / 10 in
+      let c = rc mod 10 in
+      step dist q r c (r - 1) c;
+      step dist q r c (r + 1) c;
+      step dist q r c r (c - 1);
+      step dist q r c r (c + 1)
+    end
+  done;
+  dist.(tr).(tc)
+
+;;
+
+bfs 0 0 4 4

lib/ocaml/baseline/expected.json

@@ -16,6 +16,7 @@
   "balance.ml": 3,
   "base_n.ml": 17,
   "bfs.ml": 6,
+  "bfs_grid.ml": 8,
   "btree.ml": 39,
   "brainfuck.ml": 75,
   "bsearch.ml": 7,

lib/ocaml/eval.sx

@@ -657,6 +657,14 @@
                       (error
                         (str "ocaml-eval: <- expects a field-access lhs, got "
                              (ocaml-tag-of lhs-ast)))))))
+              ;; && and || short-circuit — must NOT evaluate rhs when
+              ;; lhs already decides. Mirrors real OCaml semantics.
+              ((= op "&&")
+                (let ((lhs (ocaml-eval (nth ast 2) env)))
+                  (if lhs (ocaml-eval (nth ast 3) env) false)))
+              ((= op "||")
+                (let ((lhs (ocaml-eval (nth ast 2) env)))
+                  (if lhs true (ocaml-eval (nth ast 3) env))))
               (else
                 (ocaml-eval-op op
                   (ocaml-eval (nth ast 2) env)

plans/ocaml-on-sx.md

@@ -407,6 +407,19 @@ _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 — `&&` / `||` short-circuit fix + bfs_grid.ml
+  baseline (BFS shortest path on 5×5 grid with walls, dist 8). The
+  evaluator's `:op` handler was evaluating BOTH sides of `&&`/`||`
+  before dispatching, mismatching OCaml's left-to-right
+  short-circuit semantics. This was invisible in earlier baselines
+  because their rhs was always safe regardless of lhs, but a guard
+  like `if nr >= 0 && grid.(nr).(nc) = 0 then …` crashes with
+  "nth: list/string and number" when nr = -1 because `grid.(-1)`
+  still gets evaluated. Fix: dispatch `&&` and `||` specially in
+  `:op` to evaluate lhs first and only evaluate rhs if needed.
+  Bug latent since baseline 1; surfaced by bfs_grid with bounds-
+  checked grid access. bfs_grid optimal path 0→1→1→2→2→3→3→3→4 of
+  weighted length 8. 155 baseline programs total.
 - 2026-05-10 Phase 5.1 — tarjan_scc.ml baseline (Tarjan's
   strongly-connected components on 8-node digraph → 4 SCCs).
   Graph: 0→1→2→0 (cycle) plus 2→3, 3→4, 4→5→6→4 (cycle), 4→7.