Verify reform definitions remain valid through 2100

[MaxGhenis]

Context

The transcript review found at least one horizon-definition concern: option3 is documented as a permanent senior-deduction extension, but the active policyengine-us dependency appeared to apply the extension only through 2099-12-31. That makes 2100 convergence/matching behavior suspect.

A 75-year product needs reform definitions to be valid through 2100, not accidentally expire in 2099.

Work

• Audit each reform definition for explicit end dates, parameter horizons, and fallback behavior through 2100.
• Confirm option3 senior-deduction-extension behavior in the active policyengine-us dependency.
• Add tests that evaluate reform parameters/effects at 2099 and 2100 for all options where permanence matters.
• Patch upstream/local reform definitions or caveat affected 2100 rows.

Acceptance Criteria

• No reform described as permanent silently expires before or at 2100.
• 2100 rows are not interpreted as economic convergence when they are caused by implementation horizon limits.
• Any upstream policyengine-us dependency bug is filed or pinned with a local workaround.

Related: #81, #84, #103, #104.