Add annual additions retirement limit parameter

[MaxGhenis]

Summary

• add an IRS annual additions retirement-contribution limit parameter with explicit published anchors through 2026
• update the 401(k) elective deferral limit to the published 2026 value
• cover both values with a regression test so future uprating keeps the latest explicit IRS anchors

Testing

• /Users/maxghenis/worktrees/policyengine-us/.venv/bin/python -m pytest policyengine_us/tests/policy/baseline/parameters/test_uprating_extensions.py -q

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.49%. Comparing base (2fcac43) to head (9ebd953).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #7848       +/-   ##
============================================
- Coverage   100.00%   75.49%   -24.51%     
============================================
  Files            5     4084     +4079     
  Lines           87    58822    +58735     
  Branches         1      279      +278     
============================================
+ Hits            87    44409    +44322     
- Misses           0    14394    +14394     
- Partials         0       19       +19     

Flag	Coverage Δ
unittests	75.49% <ø> (-24.51%)	⬇️

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