feat(models): make retrieval and fusion score semantics explicit (#90)

[Muhammad-AbdullahGhani]

Related issue

Closes #90

Summary

• Add explicit ranking and score semantics across models, CLI, evidence delivery, and MCP tools:
  • SearchHit: added channel_rank, score_kind='ordering_only', score_direction='higher_is_better', score_conversion='negated_distance', distance_metric='cosine', and distance_direction='lower_is_better'.
  • FusedMoment: added combined_rank, score_kind='ordering_only', score_direction='higher_is_better', scoring_method='reciprocal_rank_fusion', contributing_channels, and channels_run.
  • FusionProvenance: added score_kind='ordering_only', score_direction='higher_is_better', and scoring_method='reciprocal_rank_fusion'.
  • EvidenceBoardCandidate: added score_kind='ordering_only' and score_direction='higher_is_better' when candidate score is present.
• Backwards compatibility: all new fields include defaults and validators so existing callers and consumers remain fully compatible.
• CLI: updated emit_search table caption and headers (Score (RRF ↑), Contributing Channels) to clarify that scores are ordering-only values rather than calibrated probabilities.
• MCP: documented in search_moments and evidence_index that candidate scores and ranks are uncalibrated ordering values where higher is better.

Validation

• pytest tests/test_search.py tests/test_search_fusion.py tests/test_query_service.py tests/test_evidence_delivery.py -p no:asyncio (37 passed)
• pytest tests/test_cli.py -k search -p no:asyncio (2 passed)
• ruff check src/vidxp/application_models.py src/vidxp/capabilities/search.py src/vidxp/search_fusion.py src/vidxp/cli_support.py src/vidxp/evidence_delivery.py src/vidxp/mcp.py tests/test_search.py tests/test_search_fusion.py (All checks passed)
• ruff format --check src/vidxp/application_models.py src/vidxp/capabilities/search.py src/vidxp/search_fusion.py src/vidxp/cli_support.py src/vidxp/evidence_delivery.py src/vidxp/mcp.py tests/test_search.py tests/test_search_fusion.py (8 files already formatted)