fix(eda): inherit KiCad reference prefix from ancestor categories - #1547

What

A part in a nested EDA category did not inherit the Reference Prefix of an ancestor category.
KiCadHelper::getKiCADPart() only consulted the part's own EDA info and then its immediate
category, so in the tree below a part placed from D-sub got refdes U? instead of J?:

PCB Components
- Connectors        (Reference Prefix = J)
  - Coaxial
  - D-sub           (blank -> should inherit J)
  - Pin Header

Why

src/Services/EDA/KiCadHelper.php:214 resolved the prefix as
part->edaInfo->prefix ?? part->category->edaInfo->prefix ?? 'U' — the category lookup was
single-level, so any deeper category lost the inherited prefix. Per #1535 the prefix should be
inherited from the next highest category that defines one.

How

Added a private getReferencePrefix(Part $part) helper and used it for the reference field:

• the part's own prefix still wins;
• otherwise the category tree is walked upwards, closest ancestor first, and the first category
  that defines a non-empty prefix wins;
• both null and '' count as "not set", so a blank prefix field inherits (the reported case);
• the 'U' fallback is unchanged when nothing in the chain defines a prefix;
• a 20-level depth guard keeps the walk bounded, matching the depth limits already used elsewhere
  in AbstractStructuralDBElement.

Only the reference field is affected — the other EDA fields and the visibility checks are
untouched.

Tests

vendor/bin/phpunit tests/Services/EDA/KiCadHelperTest.php
OK (29 tests, 68 assertions)

Four new tests in tests/Services/EDA/KiCadHelperTest.php:

• testReferencePrefixIsInheritedFromAncestorCategory — part blank, D-sub blank, parent Connectors = J -> J
• testNearestCategoryWithPrefixWins — D-sub = X, parent Connectors = J -> X
• testPartReferencePrefixOverridesCategory — part = C, parent Connectors = J -> C
• testReferencePrefixFallsBackToUWhenNothingIsSet — nothing set -> U

Mutation check

Restoring line 214 to the previous single-level lookup (leaving the new tests in place) makes the
regression test fail with exactly the reported symptom, then reverting the mutation goes green:

1) ...testReferencePrefixIsInheritedFromAncestorCategory
Failed asserting that two strings are identical.
-'J'
+'U'
Tests: 1, Assertions: 1, Failures: 1.

Fixes #1535