Repurpose ignore_default from a value parameter to a boolean flag that
combines with default or default_factory. field(default_factory=F) now
always includes the value in identifier computation (was invisible before).

- field(default=X) and field(default_factory=F): value always in identifier
- field(default=X, ignore_default=True): excluded when equal to default
- field(ignore_default=<value>): deprecated, emits DeprecationWarning
- Update refactor CLI to emit new field(default=X, ignore_default=True) syntax
- Add stability tests for all four field() default variants
- Update documentation with new API and deprecation notes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>