Task: Observation identity: point_id carries the oresmd point

Make the observation rows carry the oresmd point. point_id holds the serialised oresmd point; scalar series have a null point and the "SPOT" sentinel dies. Fixings, lineages, and the DQ artefact follow the same shape.

• market_observation.point_id is null for scalar series; "SPOT" is gone from the data and the code.
• Non-scalar points serialise the oresmd point (tenor; expiry/strike/model_subtype for vol surfaces).
• market_fixings and observation_lineages match the observation shape.
• The DQ artefact mirrors observation identity.
• Consumers (ingest loops, curve republish, editors, charts) round-trip the point.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.