[akshatXD-hash]

Closes #639

Adds src/lib/security/constant-time.ts with:

• constantTimeEqual: optional NFC normalization, UTF-8 encoding, and XOR comparison.
• timingSafeBufferEqual: full-buffer comparison with dummy-buffer handling for unequal lengths.
• timingSafeSelect: bitmask-based string selection.
• TypeScript types and JSDoc documentation.

Adds 49 Vitest tests covering equality, mismatches, unequal lengths, empty inputs, Unicode, buffer views, selection, and continued byte reads after mismatches. Integrates the suite into npm test.

Validation:

• All 96 test suites passed.
• New module achieved 100% branch, statement, function, and line coverage.
• Typecheck and secret scan passed.
• Lint completed with no errors; one warning in generated coverage output.
• No runtime dependencies added.

Security limitation:
These helpers avoid mismatch-based early exits, but JavaScript execution, normalization, allocation, and observable lengths prevent a strict constant-time guarantee. This limitation is documented.

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[TheVicky1]

Hey @akshatXD-hash ,

There are some merge conflict kindly fix it and raise a new PR

[akshatXD-hash]

I’ve opened PR #649 for this issue and resolved the merge conflicts with main. It’s ready for review.