fix(math): unify vault precision and fee boundaries - #75

Summary

• Establishes one canonical six-decimal precision policy for deposits, withdrawals, previews, and fee boundaries.
• Adds deterministic conversion metadata and a deposit preview endpoint.
• Rejects invalid, excessive, and fee-overflow inputs without mutating state.
• Adds boundary, round-trip, fee, controller, and reconciliation coverage plus a precision runbook.

Validation

• npm test — 74 passing
• git diff --check — clean
• Diff size: 502 changed lines

Issue

Closes #71