[SibiSudhanElangovan]

1. Related Issue

Closes #619

2. Summary of Changes

• Added src/lib/data/fixed-decimal.ts with fixed-precision decimal arithmetic helpers.
• Implemented addition (addFixed), subtraction (subFixed), multiplication (mulFixed), and division (divFixed).
• Added rounding modes: half-even, half-up, and truncate.
• Implemented roundFixed for rounding decimal values to a specified precision.
• Implemented formatFixed with configurable decimal precision, thousands separators, and an optional currency prefix.
• Added tests/fixed-decimal.test.ts to cover arithmetic operations, rounding behavior, formatting, exponent notation, and invalid inputs.

5. Testing & Local Validation

• TypeScript check: npm run typecheck — passed.
• Targeted tests: npm test -- fixed-decimal — passed.
• Full test suite — passed.
• npm run lint — not verified.
• node scratch/secret-scan.mjs — not verified.
• npm run build — not verified.

9. Contributor Quality Checklist

• My PR has a single, focused scope.
• My commits follow the Conventional Commits format.
• I have linked the target issue in Section 1 above.
• I reviewed my changes to ensure unrelated files were not included.
• No secrets, API keys, private tokens, or .env.local files were intentionally included in this contribution.

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