Fix bug in fields::_require_component when allocating new fields

[oskooi]

Bug: fields::_require_component() in src/fields.cpp:553 allocates new field arrays when a source is added (via alloc_f), and sets chunk_connections_valid = false, but does not set changed_materials = true. On the next step() call, step_db() lazily allocates PML auxiliary arrays (f_u) for the newly-created field components, which requires changed_materials to be true per the assertion.

Fix: (src/fields.cpp:568-573): Set changed_materials = true when _require_component allocates new fields.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.87%. Comparing base (f29a8c7) to head (73cc10a).
⚠️ Report is 120 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3188      +/-   ##
==========================================
+ Coverage   73.81%   73.87%   +0.05%     
==========================================
  Files          18       18              
  Lines        5423     5458      +35     
==========================================
+ Hits         4003     4032      +29     
- Misses       1420     1426       +6     

see 2 files with indirect coverage changes

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[stevengj]

This could be triggered if you do some timestepping, and then add a source (triggering field allocation). I think we don't see this bug currently because we virtually always create fields at the beginning and then timestep.