TOMATO-76: Stage 6 playground read-only observability contracts/docs

[cracketus]

Summary

• add Stage 6 playground read-only observability contract set (capabilities_v2, paginated_log_response_v2, pipeline_current_v2)
• add Stage 6 playground docs: compatibility matrix, observability brief, v1->v2 migration guide, and issue backlog mapping
• keep Stage 1 playground docs as explicit legacy baseline and extend docs tests to validate both legacy v1 and new Stage 6 v2 artifacts
• create Stage 6 issue backlog on GitHub (TOMATO-76 to TOMATO-83)

Testing

• ruff check tests/test_playground_bridge_docs.py docs/playground
• pytest -q

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