fix: read a mission control period the same whatever its case

[sbillinge]

A heading typed by hand says 2026Fall as readily as the 2026fall a build writes, but split_period in mc.py matched the name case sensitively, so a capitalised period was unrecognised, sorted after every real one, and took over the goals section while the archive got a section per spelling. The name is now matched whatever its case, and one spelling is kept: changes() stores a goal's period and first_period as the group's calendar spells them, and the builder and the goals lister read a stored one so. A period written some other way, such as 2026Q3, is left as it was.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (4b34f3c) to head (1db53e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1390      +/-   ##
==========================================
+ Coverage   91.23%   91.29%   +0.05%     
==========================================
  Files          29       29              
  Lines        3308     3330      +22     
==========================================
+ Hits         3018     3040      +22     
  Misses        290      290              

Files with missing lines	Coverage Δ
tests/test_mc.py	100.00% <100.00%> (ø)
tests/test_mc_changes.py	100.00% <100.00%> (ø)
tests/test_mc_listers.py	100.00% <ø> (ø)
tests/test_missioncontrolbuilder.py	100.00% <100.00%> (ø)

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