Stage 6: make explicit report definitions and run metadata canonical

[anth-volk]

Stage 6 of the report-output migration on policyengine-api.

Scope:

• accept and persist explicit report_spec payloads for new reports
• key report creation/reuse by canonical report identity instead of the old thin (country, simulation_1, simulation_2, year, api_version) rule
• resolve report reads through canonical parents plus display-run selection
• preserve explicit report definitions across later PATCH/sync traffic
• preserve execution metadata from existing runs instead of rebuilding it from mutable parent rows
• validate explicit aliases by canonical report identity

Acceptance criteria:

• distinct explicit economy report specs create distinct report rows
• aliased reads resolve through canonical parents without recreating current-version parent rows
• later PATCH/sync does not silently downgrade explicit report specs
• later PATCH/sync does not drift stored run metadata unless explicitly overridden
• old malformed legacy rows remain readable in compatibility mode

Non-goals:

• historical backfill of report_identity_* fields and rebuilt alias rows (handled separately in the updated stage 4 script)
• frontend UI changes to view older runs
• pinned run behavior