ocaml: phase 5.1 gas_station.ml baseline (circular tour start = 3)

Classic O(n) greedy gas-station algorithm:

  walk once, tracking
    total = sum of (gas[i] - cost[i])  -- if negative, no answer
    curr  = running tank since start   -- on negative, advance
                                          start past i+1 and reset

  if total < 0 then -1 else start

For gas = [1;2;3;4;5], cost = [3;4;5;1;2], unique start = 3.

Tests `total` + `curr` parallel accumulators, reset-on-failure
pattern.

202 baseline programs total.

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.
+- 2026-05-11 Phase 5.1 — gas_station.ml baseline (find unique start
+  station for circular gas tour, gas=[1;2;3;4;5] cost=[3;4;5;1;2]
+  → start at index 3). Classic O(n) greedy: walk once tracking the
+  total tank delta (if negative, no solution → -1) and a running
+  tank that resets to 0 when it goes negative, advancing start
+  past the failing index. From station 3: tank 3,3+5,3+5−3,…
+  succeeds. Tests `total` + `curr` parallel accumulators, reset-
+  on-failure pattern. 202 baseline programs total.
 - 2026-05-11 Phase 5.1 — min_jumps.ml baseline (greedy BFS-like
   min jumps to reach end of [2;3;1;1;2;4;2;0;1;1] = 4). At each
   position track the farthest reach within the current "BFS