Add synthetic financial ledger fixture with accounting negatives (#496)

[muhammad-muneeb3]

What changed?

• Added a new synthetic test fixture tests/fixtures/financial_ledger.csv and its accompanying expectations file tests/fixtures/financial_ledger.expectations.json[cite: 1, 3].
• Extended parse_currency_parts in src/freshdata/semantic/experts.py to identify and parse parenthesized accounting negative values (e.g., (1,250.00)) as valid negative numeric values[cite: 1, 2].
• Added financial identifier checks via _MONEY_NAME in src/freshdata/engine/context.py and src/freshdata/semantic/context.py so amount and balance columns are not misclassified as id or text roles due to high cardinality[cite: 2].
• Registered financial_ledger in tests/expectations.py, generated the golden snapshot, and added targeted test coverage in tests/test_currency_locale.py[cite: 1, 2, 3].

Why?

Ensures FreshData robustly handles localized financial conventions—specifically parenthesized accounting negatives and currency formats—without regressing semantic detection or data cleaning pipelines[cite: 1].

Fixes #496[cite: 1]

How was it tested?

• New unit tests added in tests/
• Ran fast CI lane locally: pytest -m "not online and not large"
• Ran linting and type checks: ruff check . and mypy src/freshdata.

Targeted test command run locally:

pytest tests/test_currency_locale.py tests/test_realworld.py -k financial_ledger tests/test_golden.py -k financial_ledger tests/test_experts_guard_mutants.py -k currency --no-cov

## Any performance impact?

<!-- Will this change affect wall-clock runtime, memory allocation, or startup time? If yes, provide timings. -->

- [x] None / negligible
- [ ] Measured with `benchmarks/bench.py` (details below):

## Any compatibility concerns?

<!-- Does this change public API signatures, default behavior, or supported Python/pandas versions? -->

- [x] None / fully backward-compatible
- [ ] Deprecation or behavior change documented below:

## Documentation updated?

<!-- If user-facing behavior changed, did you update docs/, examples/, or CHANGELOG.md? -->

- [ ] Documentation updated in `docs/`
- [ ] Examples verified or updated
- [x] Note added under `[Unreleased]` in `CHANGELOG.md`

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