Register Matlab .mat format with MAT.jl

[ViralBShah]

Summary

• Registers .mat extension with MAT.jl (UUID 23992714-dd62-5051-b70f-ba57cb901cac) so that load("file.mat") and save("file.mat", data) work via FileIO
• Fixes register Matlab's .mat #361

Test plan

• CI passes
• using FileIO; query("test.mat") returns File{format"MAT"}

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (4e544bd) to head (ed4b3e4).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   91.44%   91.44%           
=======================================
  Files          11       11           
  Lines         713      713           
=======================================
  Hits          652      652           
  Misses         61       61           

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