test(fx): address CodeRabbit nits on conservation tests

[GeiserX]

Addresses both CodeRabbit findings on #260 (test-only, no production change):

1. Decimal-safe test inputs — the Op helper tuples typed amounts/rates as JS number; now decimal strings (Money = string), honoring the repo's Financial Precision rule (never Number for amounts/rates). toEvents already wraps each in new Decimal(...), so all figures are numerically identical (160.00/320.00/680.00… unchanged).
2. Diagnostic-only test now covers both branches — its title says "whether or not a mismatch is emitted" but it only tested the clean run. It now forces a real fx.conservation_mismatch via BreakableEngine and asserts the disposals/gain stay byte-identical after emission.

Verified: typecheck:tests clean, lint clean, conservation suite 19/19. No engine or tax-figure change.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 754113d5-63e1-4993-8b7c-31fa41e28507

📥 Commits

Reviewing files that changed from the base of the PR and between b993a19 and b7a0b2d.

📒 Files selected for processing (1)

• tests/engine/fx-conservation.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/fx-conservation-test-nits

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