fix(test): attach caplog handler to non-propagating explainer logger

test(llm): tighten live integration test (slow marker, log-gated skip, on-topic assertion)

test(llm): add network-gated end-to-end OpenRouter integration test

test(llm): pin 401 short-circuit + 400 try-next-model behavior (red)

feat(llm): modality dispatch — explain(payload, modality) for BBB/EEG/MRI

feat(llm): explainer with deterministic template + OpenRouter fallback