Fix negative HI rates in balanced-fix endpoint solves #104

Context

The transcript's external reviews identified a substantive special-case failure: the option13 2100 balanced-fix raw backfill implies negative HI payroll-tax rates, which is not economically plausible and likely contaminates option14_stacked at 2100 because it is chained off the balanced-fix baseline.

This is more specific than the general 2100 endpoint issue: it is a solver/constraint failure for balanced-fix HI rates.

Work

• Rerun or debug the option13 2100 balanced-fix solve and inspect solved OASDI/HI payroll-tax rates directly.
• Enforce nonnegative payroll-tax-rate constraints where required by policy definition.
• Rerun option14_stacked 2100 after fixing the balanced-fix endpoint.
• Add a validation check that fails any special-case raw row with negative tax rates unless explicitly allowed and documented.

Acceptance Criteria

• option13 2100 does not rely on negative HI payroll-tax rates.
• option14_stacked 2100 is regenerated from the corrected balanced-fix endpoint.
• Special-case endpoint rows pass a rate-bound validation gate before publication.

Related: #80, #81, #93, #103.