v0.3.0 - EEGDash backend + evaluation provenance

v0.3.0 adds interoperability with EEGDash (the new OpenNeuro/NEMAR catalog) and an
evaluation-provenance record - complementary to EEGDash's discovery/loading, not a
duplicate of it.

Added

• A pluggable DatasetBackend protocol. The existing MOABB logic is exposed via a
  moabb backend; a new eegdash backend reads EEGDash metadata and record listings
  with no signal download.
• bcicards scan --backend eegdash <dataset-id> produces:
  • a metadata card (modalities, record/subject counts, license, DOI, BIDS validation
    status, and honest leakage risk factors - not a "RISK: HIGH" verdict), and
  • <id>.provenance.json (schema bci-evaluation-card/0.1) recording the exact
    cohort (subjects/sessions/records) so an evaluation can be reproduced.
• New optional card fields, an [eegdash] install extra, and a design spec at
  docs/design/eegdash-backend.md.

Scope / honesty

• The eegdash backend is metadata-only by design. No baseline is run for EEGDash
  datasets (they span many modalities/tasks); that is intentionally deferred.
• EEGDash is new and its API may change; the backend is isolated with a small surface.

Quality

• 24 tests (EEGDash client mocked - no network in CI); ruff lint + format enforced; CI
  green on Windows/Linux x Python 3.11/3.12. Still not medical software; no diagnostic claims.