Add US PUF support channel expansion

[MaxGhenis]

Summary

• add a US PUF support-channel frame transform that clones every US entity table into baseline ASEC and PUF tax-detail channels
• remap structural IDs/foreign keys for cloned rows while preserving entity-prefixed source IDs, support channel, and clone index provenance
• split typed weights evenly across support channels so support expansion increases row support without doubling population mass
• declare the frame-structural support stage immediately before the PUF tax-detail donor stage

Validation

• uv run ruff check packages/populace-build/src/populace/build/us/__init__.py packages/populace-build/src/populace/build/us/puf_support.py packages/populace-build/tests/test_us_plan.py packages/populace-build/tests/test_us_puf_support.py
• uv run --package populace-build --extra us pytest packages/populace-build/tests/test_us_puf_support.py packages/populace-build/tests/test_us_plan.py
• uv run --package populace-build --extra us pytest packages/populace-build/tests

Review

• cycled through read-only subagent review; first pass found a missing baseline-channel invariant, fixed with a guard/test; second pass found no actionable issues.