Add Ledger target profile contracts

[MaxGhenis]

Summary

• add packaged Ledger target profile dataclasses and loader
• add a UK local-geography profile with source-backed measurement contracts and policyengine/axiom bindings
• update Ledger/Populace boundary docs so Ledger owns target profiles and measurement contracts while Populace owns support-aware activation and calibration execution

Notes

• UK source selectors are target contracts ahead of complete arch-uk source-package parity; values are still resolved from Ledger facts, not embedded in the profile.
• The profile validator rejects target-value-like keys recursively while allowing legitimate filter predicate thresholds.

Tests

• uv run python -m json.tool policyengine_ledger/target_profiles/uk_local_geography.json >/dev/null
• uv run python -m pytest tests/test_policyengine_ledger_target_profiles.py -q
• uv run ruff check policyengine_ledger/target_profiles policyengine_ledger/targets tests/test_policyengine_ledger_target_profiles.py
• uv run ruff format --check policyengine_ledger/target_profiles policyengine_ledger/targets tests/test_policyengine_ledger_target_profiles.py
• git diff --check

Review cycle

• Read-only subagent review found hidden-value guard gaps; fixed with recursive/context-sensitive validation and regression tests.
• Final read-only subagent review: no actionable findings.