fix: reject non-concrete eq/ne/not in const evaluation

[s0mnathh]

Summary

Reject non-concrete equality, inequality, and logical-not operations during constant evaluation. Add a regression test for !F::FLAG on generic impl constants and report E2127 instead of folding the expression.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

Generic impl constants are not fully concrete during constant evaluation, so ==, !=, and ! should not be folded eagerly. Before this change these operators could bypass the existing non-concrete checks and produce an incorrect boolean result instead of a diagnostic.

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What was the behavior or documentation before?

Constant evaluation could accept equality, inequality, or logical-not expressions over generic impl constants and fold them as if the value were known. This could silently produce the wrong constant result instead of reporting E2127.

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What is the behavior or documentation after?

The evaluator now reports E2127 for ==, !=, and ! when any argument is a non-concrete impl constant. The regression test covers !F::FLAG and verifies that unsupported constant expressions are rejected.

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Related issue or discussion (if any)

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Additional context

This follows the same constant-evaluation policy already used for other unsupported operations on generic impl constants and keeps the fix scoped to the boolean operator fast path.

[reviewable-StarkWare]

This change is 

[orizi]

@orizi reviewed all commit messages and made 1 comment.
Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on s0mnathh).

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a discussion (no related file):
fixed by #9879