ocaml: phase 5.1 tarjan_scc.ml baseline (8-node digraph, 4 SCCs)

Tarjan's strongly-connected components in a single DFS using
index/lowlink:

  graph (8 nodes, directed):
    0 -> 1 -> 2 -> 0   (3-cycle)
    2 -> 3
    3 -> 4
    4 -> 5 -> 6 -> 4   (3-cycle)
    4 -> 7

  SCCs: {0,1,2}, {3}, {4,5,6}, {7}  =  4 components

Module-level ref + array state (index_arr, lowlink, on_stack,
stack, scc_count). When lowlink(v) = index(v), pop from stack
until v is removed; that's a complete SCC.

Tests: recursive function with module-level mutable state,
nested begin/end branches inside List.iter closure, inner
`let rec pop ()` traversing a ref-of-list, pattern match on
[] / h :: rest cons-list shape.

154 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -135,6 +135,7 @@
   "stable_unique.ml": 46,
   "subseq_check.ml": 3,
   "tail_factorial.ml": 479001600,
+  "tarjan_scc.ml": 4,
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "topo_sort.ml": 6,