v2.20.0 — Architecture: four module-deepening refactors

v2.20.0 (2026-09-10)

Architecture: four module-deepening refactors

Implements all four candidates from the codebase architecture review.
All changes are additive or internal — no public interface breaks.
42 new unit tests, all passing.

Candidate 1 — extract_vlp_source_ids (locality)

Extracted the VLP source-ID extraction logic — previously duplicated verbatim
in _execute_var_length_labeled and _execute_var_length_labeled_path_funcs —
into a single free function in query.py. Three extraction paths (label-based,
direct node_id = ?, full-SQL DISTINCT) live in one place with explicit
documentation and 10 dedicated unit tests. The $C(0) bug that required 3 hours
to debug in session is now a 5-line unit test.

Candidate 2 — IRISGraphEngine sub-namespace attributes (depth)

Added engine.graph, engine.cypher, engine.temporal, and
engine.algorithms as thin-delegator namespace attributes. The 192 top-level
methods are unchanged (additive only). Callers who only do graph mutations can
discover the 11-method engine.graph interface rather than scanning 192 methods.
Test fixtures can mock a 6-method sub-engine instead of a 192-method engine.

Candidate 3 — commit() transport seam (locality)

Extracted _send_changeset(cs) -> _CommitWireResult from GraphLedger.commit().
Transport concerns (JSON chunking, ObjectScript call, wire-response parsing) are
now isolated from commit-semantics concerns (NKG dirty-marking, post_commit SQL,
metrics). _CommitWireResult is a plain dataclass — no exceptions raised in
transport. 8 unit tests cover each concern independently with mock _call.

Candidate 4 — BFS strategy protocol (leverage)

Added _BfsStrategy internal protocol and three concrete adapters
(_ArnoBfsAdapter, _ObjectScriptBfsAdapter, _SqlBfsFallbackAdapter) inside
iris_sql_store.py. execute_bfs now delegates to _select_bfs_strategy().
The three implicit adapters are now named, typed, and independently testable.
Adding a fourth BFS acceleration path is one new adapter class. 11 unit tests.