Try to widen named types during box adaptation

[Linyxus]

fixes #19076

[Copilot]

Pull Request Overview

This PR addresses issue #19076 by improving type adaptation during box adaptation by widening named types.

• Adds a test case in the captures suite to validate impure capture checking.
• Modifies the CheckCaptures component to introduce a widening strategy for named types and a fallback check using the widened type.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/pos-custom-args/captures/cc-tvar-impure.scala	Adds a test scenario to assess correct type capture handling.
compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala	Implements widening of named types with helper functions and fallback logic during box adaptation.

Comments suppressed due to low confidence (1)

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala:1338

• [nitpick] Consider renaming 'tryThisType' to 'isInitiallyCompatible' to more clearly convey that the function checks the initial compatibility before attempting widening.

def tryThisType: Boolean =