Import: document EDA/KiCad columns, add regression tests, fix eda_invisible inversion

[Sebbeben]

Summary

While importing parts with KiCad/EDA columns I hit some rough edges around the EDA import fields. This PR
does three small things: documents the EDA import columns (they were undocumented), locks in the two
supported column forms with tests, and fixes one alias that stored the wrong value.

1. Document the EDA / KiCad import columns

The eda_* import columns weren't documented anywhere, which made the flat names hard to discover. Added a
section to docs/usage/import_export.md listing them, and noting that the importer accepts both:

• the flat form (eda_kicad_symbol, eda_value, … and short aliases like kicad_symbol), and
• the nested eda_info.* form used by the CSV/JSON export and by the shipped part_import_example.csv
  (eda_info.kicad_symbol, …).

2. Regression tests for both column forms

The nested eda_info.* form already works today (the CSV encoder un-flattens the dotted headers and the
eda_info embeddable is in the import serialization group), but it wasn't covered by a test — so an export →
re-import round-trip, and the EDA columns in the example CSV, could silently regress. Added to
EntityImporterTest:

• testImportAcceptsNestedEdaInfoColumns — eda_info.kicad_symbol / …footprint / …reference_prefix /
  …value fill the part's EdaInfo.
• testImportAcceptsFlatEdaColumns — the flat form and its aliases still work.

3. Fix: eda_invisible stored the inverse value

eda_invisible was mapped to eda_visibility by a plain key-rename, so eda_invisible=1 set
visibility=true — i.e. it made the part visible, the opposite of what the name says. It's now handled
explicitly and inverted, so eda_invisible=1 correctly hides the part. Covered by
testImportInvisibleAliasInvertsVisibility.

Notes

No schema/behaviour change beyond the eda_invisible fix; the nested-column support was already present and is
only being documented and tested here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.50%. Comparing base (523b07e) to head (16f0c4d).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1436      +/-   ##
============================================
+ Coverage     58.49%   58.50%   +0.01%     
- Complexity     8701     8703       +2     
============================================
  Files           638      638              
  Lines         28123    28125       +2     
============================================
+ Hits          16451    16455       +4     
+ Misses        11672    11670       -2     

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