fix(classes): keep inner class references ($) from UTF-8 pool in ImportVisitor

[elharo]

Fixes #140

The UTF-8 constant pool filter in ImportVisitor (VALID_UTF8_PATTERN) rejected strings containing $, so inner class references like org/apache/tools/ant/XmlLogger$TimedElement were silently dropped from the discovered imports. Additionally, QUALIFIED_IMPORT_PATTERN did not allow $, causing raw descriptor strings (e.g. L...$Inner;) to leak in unparsed.

Changes:

• allow $ in VALID_UTF8_PATTERN
• allow $ in the qualified-class portion of QUALIFIED_IMPORT_PATTERN so descriptors are still unwrapped correctly
• add a regression test that compiles a class holding an inner-class reference in its UTF-8 pool and asserts it is collected as an import

All 76 tests pass; checkstyle clean.