feat(mutation): Implement AST Mutation Testing Engine to Validate Rule Precision

[SharifIbrahimDev]

Closes #607

Summary of Changes

• Built AST mutation testing framework in tests/mutation/ast-mutator.ts to automatically inject gas anti-patterns (uint8 type sizing, uncached loop length, memory over calldata, postfix increment, redundant SLOADs).
• Built MutationRunner (tests/mutation/mutation-runner.ts) to execute mutated code variants against the GasGuard rules engine and track mutant detection.
• Generates structured MutationReport displaying mutant kill score percentages and survived mutant details.
• Added unit test suite in tests/mutation/mutation.spec.ts.

Files Added

• tests/mutation/ast-mutator.ts
• tests/mutation/mutation-runner.ts
• tests/mutation/index.ts
• tests/mutation/mutation.spec.ts

Acceptance Criteria Checklist

• Mutation framework generates valid mutated code variants automatically.
• Reports rule detection rate ("mutant kill score") across the rule test suite.
• All 3 unit tests passing.

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