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.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _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 — 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.
+  SCCs: {0,1,2}, {3}, {4,5,6}, {7} = 4 components. Single DFS
+  with index/lowlink, on-stack flag, pop until root when
+  lowlink = index. Tests recursive functions with module-level
+  ref + array state, nested begin/end branches inside List.iter
+  closure, inner `let rec pop ()` walking ref-of-list, pattern
+  match on `[] | h :: rest`. 154 baseline programs total.
 - 2026-05-10 Phase 5.1 — lev_iter.ml baseline (iterative
   Levenshtein DP, sum of 5 distances = 16). Rolling-array DP
   (O(min(m,n)) space). Distances: kitten→sitting=3, saturday→