refactor: improve performance, realiability and remove unsafe code

[spinualexandru]

Changes:
Removed unsafe code:

• MutableCategoryInstance no longer holds document reference
• Clean borrow checker compliance without raw pointers

Clone reduction

• use std::mem::take in config.rs instead of .clone() for error collection

Misc

• Added strip_prefix/strip_suffix instead of manual slicing
• Added HandlerFn type alias for complex handler functin type

Tests:
Added edge case tests for:

• Color parsing: invalid hex, wrong length, out of rang values, malformed syntax
• Vec2 parsing: non numeric, missing components, whitespace, negative, floats
• General: Empty values, unicode, special chars
  Round trip serialization tests:
• All types round trip (vars, values, handles, special cats)
• Handler removal with document sync verification
• Special category removal with doc sync verification
• Nested categories round-trip