feat(precompute): install ingest, schema, and producer contracts

[zzylol]

Summary:

• Make PrecomputePlan the authoritative runtime ingest/schema/producer contract installed with the PhysicalPlan.
• Describe /v1/metrics identity requirements, allowed producers, exact state family/parameters, source/value column, SID grouping layout, window shape, and supported encodings.
• Cross-validate every schema against BackendPlan during compile and atomic install.
• Deduplicate shared physical materializations by policy fingerprint so multiple readouts can reuse one maintained state without failing plan validation.
• Rebased on feat(plan): add versioned staged activation #474 and the node-bound DAG executor in feat(query): execute authoritative QueryPlan #472.

Verification:

• cargo check -p control_plane -p data_plane
• shared-materialization regression test passes
• tampered-schema cross-plan validation test passes
• git diff --check

Stack 3/N. Please do not merge independently of #472/#474.