Port US PUF real-half and clone support diagnostics

Source backlog

Copied from the still-relevant parts of the policyengine-us-data backlog:

• PolicyEngine/policyengine-us-data#1139: PUF clone half receives zero final weight.
• PolicyEngine/policyengine-us-data#1158 / PR PolicyEngine/policyengine-us-data#1159: enforce PUF-clone weight-share floor and tax-pathology cap before upload/publication.
• PolicyEngine/policyengine-us-data#1170: separate PUF real-half QRF imputation from clone-half rich-preserving QRF, with income-conditioned predictors.
• Related: PolicyEngine/policyengine-us-data#555, #560, #590, #595, #541.

Populace direction

Translate the useful behavior into Populace's source-plan architecture rather than porting old staged-pipeline code.

• Country content may declare support contracts, predictors, support floors, and smoke expectations in specs/manifests.
• Shared Populace runtime code should implement any generic real-half / clone-half imputation operation.
• No policyengine-us-data, Microplex, or L0 implementation should be copied directly.

Acceptance criteria

• Populace has a US support diagnostic for PUF real-half vs clone-half weight share and tax-pathology share.
• A release/publish gate rejects missing or pathological clone support before artifact publication.
• PUF-imputed tax-detail variables have fixture/smoke tests preventing known explosions such as charitable/business/tax-detail outliers.
• The support contract is manifest/spec-driven and covered by unit tests.

Part of #46.

Start by locating Populace's source-plan specs/manifests, shared runtime imputation operations, release or publication gate, and existing fixture, smoke, and unit-test entry points. Define the manifest-driven support contract for PUF real-half and clone-half behavior, then cover weight-share and tax-pathology diagnostics, clone-support rejection, and imputed tax-detail regression cases. Done means the contract and tests enforce all listed acceptance criteria without copying policyengine-us-data, Microplex, or L0 code.