Fix bug in zero_fields_near_cylorigin for |m| >= 2

[oskooi]

Fixes #2644.

Root cause: The zero_fields_near_cylorigin stability mechanism in step_db.cpp (lines 397-456) is incomplete for |m| >= 2. It only zeros field components on integer-r grid points (Dp, Dz for D_stuff; Br for B_stuff) but not the half-integer-r components (Dr for D_stuff; Bp, Bz for B_stuff).

When an Er source extends to r=0, it injects current into Dr at the half-integer-r grid points near the origin (r = 0.02, 0.06 for resolution 25). Because Dr was not being zeroed, the nonzero Dr/Er at small r feeds into the Bz curl update via the im/r × Er term, which amplifies by ~100× at r=0.02 for m=2. This Bz then drives Hz, which feeds back into Dr via the im/r × Hz term — creating an exponentially growing feedback loop.

When offset = 0.15 (in the test script), the source region starts at r=0.075, placing no source current at the first two Er grid points (r=0.02, 0.06) that fall within the zeroing range, so the instability doesn't trigger.

Fix (src/step_db.cpp): Extended the field-zeroing to include all six D/B components near the cylindrical origin for |m| >= 2:

• D_stuff: Added zeroing of Dr alongside existing Dp, Dz
• B_stuff: Added zeroing of Bp, Bz alongside existing Br

This breaks the feedback loop by ensuring Bz is zeroed near r=0, so Hz remains zero and the im/r amplification never feeds back into Dr.

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✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3164      +/-   ##
==========================================
+ Coverage   73.81%   73.90%   +0.09%     
==========================================
  Files          18       18              
  Lines        5423     5454      +31     
==========================================
+ Hits         4003     4031      +28     
- Misses       1420     1423       +3     

see 2 files with indirect coverage changes

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