stitchgraph v2.3.0 — shared Tarjan SCC core

stitchgraph v2.3.0 — shared Tarjan SCC core

A small, internal de-duplication release. No API, schema, or behaviour change: indexes,
scan cycle detection, and find_data_loops produce byte-identical results. Indexes do not need
rebuilding.

Changed

Shared tarjan_scc helper

The strongly-connected-components algorithm was duplicated verbatim in two places:

• core/reach.py → strongly_connected_components (call / import cycles, behind scan)
• core/dataloop.py → _tarjan (data-feedback loops)

Both copies carried an identical strongconnect implementation and recursion-limit handling. They
are now a single core/_scc.py:tarjan_scc(adj, seeds, node_count). Each call site keeps its own
adjacency construction, seed set (reach: all node ids; dataloop: adjacency keys), and
post-filter — so behaviour is preserved exactly, including the temporary recursion-limit raise that
is always restored in a finally (never leaked to the host).

This was the one piece of genuine redundancy surfaced by the matrix-as-oracle research line
(research/, not part of the package): the body-level structural-clone detector found the
byte-identical duplication the call-graph detector was blind to, and an independent
program-dependence-graph pass re-found the same pair.

Added

• tests/test_scc.py — 15 direct unit tests pinning the shared primitive: component identity
  (empty / single / self-loop / chain / 3-cycle / two independent cycles / cross-edge into an
  already-finished SCC), destination-only and out-of-adjacency seeds, reverse-topological
  ordering, a 5 000-node chain (no RecursionError), defaultdict non-mutation, and
  recursion-limit restoration on both normal return and an exception mid-walk. Stdlib-only, so it
  runs in the core-only CI job.

Quality gate

• ruff + mypy clean; full suite 590 passing; differential oracle suite (27) green.
• Mutation meta-oracle on tarjan_scc: 6/6 mutants killed by tests/test_scc.py alone.
• Two-round full-diversity adversarial panel (opus / sonnet / haiku), clean. The panel
  confirmed component / ordering / recursion-limit equivalence to the old inline copies — including
  a 650-graph random differential with zero divergences between old and new — and verified that
  find_stale liveness is structurally independent of SCC results (SCC feeds only advisory
  cycle / data-loop findings, never stale rooting), so the cardinal rule is insulated from this
  change. Round 2 surfaced a unit-test coverage gap (the on_stack cross-edge guard), which was
  closed before release; the two confirming rounds were clean.

Notes

The companion research (research/SYNTHESIS.md) and the larger deferred direction — an
intra-procedural (body / CFG / DFG / PDG) matrix, a candidate v3.0.0 feature — are documented in
docs/IDEAS.md §5. This release ships only the safe, validated de-duplication.