Encode statutory IRMAA threshold indexing for years after 2026

Split from #8787 (closed; 2026 IRMAA schedules verified against SSA POMS HI 01101.020).

The Part B/D IRMAA MAGI thresholds carry no uprating metadata, so years after 2026 hold the 2026 values. The statutory rule (42 U.S.C. § 1395r(i)(5)) indexes the thresholds to CPI-U over the 12 months ending August, rounded to the nearest $1,000, with the top ($500k/$750k) bracket following its own schedule (frozen through 2027, indexed after).

Holding the last official year was chosen over approximating because a wrong projection is worse than a documented lag — but once CMS publishes 2027 values (or for projection use-cases), encode either the official values or the exact statutory indexation. The special top-bracket handling means a naive `uprating:` stanza is not sufficient.

Start by tracing the existing Part B/D IRMAA MAGI threshold definitions and their uprating metadata, then read the statutory rule in 42 U.S.C. § 1395r(i)(5) alongside SSA POMS HI 01101.020. Done means years after 2026 use the official values or exact CPI-U indexation, including the separate frozen-through-2027 and later-indexed top bracket schedule.

Written by the indexing model from the issue text.