fix(types): honor generic variance for same-class assignments

[chess10kp]

Closes #8550

Also fixes the same mutable-container covariance hole reported in #8580.

Summary

TypeEvaluator._assign_class already applied declared variance when comparing a generic subclass with a generic base, but the same-class path always used one-way assign_type. This accepted list[Dog] as list[Animal] even though list is invariant.

This patch centralizes generic argument comparison and applies:

• covariant: source argument assigns to destination argument
• contravariant: destination argument assigns to source argument
• invariant: both directions must assign

The helper is used by both same-class and generic-base comparisons. Unknown destination arguments remain open for inferred tuple filters.

Verification

• Minimal repro before the fix: jac check accepted list[Dog] passed to list[Animal].
• After the fix: the repro reports E1053: Cannot assign list[Dog] to parameter 'xs' of type list[Animal].
• Targeted test: cd jac && jac test tests/compiler/passes/test_checker_bugs.jac --test_name bug_8550_same_class_generic_variance -x (1 passed).