Three-substrate agreement test: BERT validate_mode ⇔ GSR lens constraints ⇔ Lean hypotheses

[rsthornton]

Deferred from bert#88 (PR #92). The kernel-mode preconditions now exist in three substrates and should be cross-checked so they can never silently drift:

1. Lean — HasBond / Irreflexive hypotheses in systems-science-foundations/Systems/Klir/ViewGeneration.lean
2. GSR (Python) — enforce_constraints in general-systems-reasoner/.../modes/bunge.py + klir.py (shipped/deployed 6/11)
3. Rust — bert-core::validate::validate_mode (this PR)

The check

A model that BERT's validate_mode(_, Structural) rejects as an aggregate (no bond) should be the same model GSR's Bunge lens calls an aggregate; likewise the Operational self-loop / Mobus k≠o rule. Build a shared fixture set and assert the three substrates agree on accept/reject.

Why deferred

bert#88 was scoped bert-core-only (Rust) to keep the last-session-of-day diff small. This pulls in the GSR Python repo, so it's its own task.

Acceptance

• Shared aggregate fixture: BERT validate_mode(_, Structural) errors ⇔ GSR Bunge lens flags aggregate.
• Shared self-loop fixture: BERT validate_mode(_, Operational) errors ⇔ GSR Mobus/Klir constraint flags it.
• A note in the bert-dev "Validators" doc pointing to all three sites.

Refs bert#88