fix(calculations): align solver with multi-person projections & fix OAS calculations

[halfguru]

This PR fixes two major calculation issues in the retirement engine:

1. Household OAS Calculation:

   • Previously, SummaryCard calculations did not account for OAS at all, while IncomeBreakdownCard evaluated a single hardcoded OAS max of $9,024 when the household retirement age reached 65+.
   • Moved the OAS calculation into ProjectionContext. It dynamically verifies eligible seniors based on their age at the household retirement point (meaning a dual-income household where both turn 65+ during retirement gets two OAS estimates, which is correct).
   • Aligned all summary metrics and breakdown segments to draw from this single source of truth.

2. Additional Savings Solver Synchronization:

   • Previously, the solver called the WASM calculate_additional_annual_savings method using simple parameters, which assumed continuous savings and compound growth for all accounts until retirement.
   • For multi-person households where one member retires early (e.g. at 58) and freezes their contributions, the WASM solver simulated a much higher ending balance ($3.1M instead of the actual $1.56M), resulting in incorrect $0/year additional savings recommendations when a gap actually existed.
   • Swapped the solver to use the exact mathematical sinking fund formula based directly on the actual computed projection gap, aligning the recommendations perfectly.