[codex] Start ingestion v2 architecture

[FrodeHus]

Closes #79

Summary

Starts the greenfield ingestion v2 implementation:

• adds the implementation plan under docs/superpowers/plans/
• adds typed ingestion observation entities and EF configurations
• adds SoftwareSourceIdentity and SoftwareRelease
• adds IngestionRunDelta for run-scoped downstream work
• adds IObservationBulkLoader and normalized observation input records
• adds PostgreSQL temp-table + binary COPY observation loader
• adds IIngestionStateMerger with the first software merge stage
• wires ingestion v2 services into DI

Notes

The EF migration is intentionally not included. Per project workflow, Frode will run:

dotnet ef migrations add GreenfieldIngestionV2 \
  --project src/PatchHound.Infrastructure \
  --startup-project src/PatchHound.Api

until then, any Testcontainers test path that calls MigrateAsync() will fail with EF pending model changes.

Validation

• dotnet build PatchHound.slnx
• dotnet test PatchHound.slnx --filter FullyQualifiedName~IngestionV2 -v minimal

Full dotnet test PatchHound.slnx -v minimal is blocked until the migration is generated because the model intentionally has pending changes.