Deprecate split state/district H5 pipeline after Populace replacement

[MaxGhenis]

Populace is moving to a single national H5 with state and congressional-district region scoping in policyengine.py rather than one-file-per-state and one-file-per-district artifacts. Once the new Populace release is certified into policyengine.py, this repo should stop presenting states/*.h5 and districts/*.h5 as the live path.

Tracking context:

• Replace all remaining policyengine-us-data artifacts with Populace populace#204 is the central replacement tracker.
• Publish US releases as a single national H5 populace#237 removed state/district artifact generation from Populace releases.
• Current downstream split-H5 consumers are listed in Replace all remaining policyengine-us-data artifacts with Populace populace#204 comment: Replace all remaining policyengine-us-data artifacts with Populace populace#204 (comment)

Cleanup checklist:

• Mark state/district H5 production as legacy or archived after Populace certification.
• Stop publishing new split regional H5 files unless explicitly needed for historical reproducibility.
• Keep enough manifests/docs for old releases to remain reproducible.
• Redirect docs and examples to the certified policyengine.py Populace bundle and region/scoping APIs.
• Remove or quarantine release-promotion tests that assume states/ and districts/ are active release outputs.