Reject misplaced uprating rounding metadata

[anth-volk]

Fixes #216

Merge order: PolicyEngine/policyengine-us#9371 must merge before this pull request. Current PolicyEngine US main contains 176 definitions that this validation will reject.

Summary

• reject metadata that places rounding and uprating at the same level
• report that rounding must be nested inside uprating because the previous structure is ignored
• validate ordinary parameters, scale-level metadata, and bracket-level metadata through the common parameter schema check
• retain the existing supported nested rounding behavior

Validation

• uv run --frozen pytest tests/core/parameters -v — 49 passed
• uv run --frozen ruff format . — 270 files unchanged
• uv run --frozen ruff check . — all checks passed
• loaded the corrected PolicyEngine US parameter tree through this validation — 75,480 parameter descendants loaded

The full Core test suite was not run locally; the complete parameter suite covering this loader behavior passed. make format could not locate ruff on the shell path, so the same configured formatter and lint checks were run through the repository's uv environment.