Implement S34 PyPI hosted format

Implement the S34 PyPI hosted format per the spec and plan: schema, config, validation, metadata extraction, transport/resolver/middleware, datastore, routing/auth, Simple Repository API (HTML + JSON), publish flow, downloads + PEP 658 metadata sidecars, observability, and the pip/twine/poetry/uv conformance harness.

Tracks the implementation MR that lands the 22 plan steps. Stacks on the spec MR and plan MR.

Follow-up work is tracked separately (PEP 740 attestations: #363).