wire prefixMode from RPN into Operations for metric-aware arithmetic

Add prefixMode field to Operations struct with thread-safe Get/Set
accessors, propagating the user-set prefix mode (SI/IEC) from the RPN
parser through the evaluate path.

- Replace all hardcoded SI arguments in convertToBase/convertFromBase
  calls with o.GetPrefixMode() across arithmetic, comparison, and
  convert operations
- Cache GetPrefixMode() once per operation to avoid repeated mutex
  overhead
- Sync r.ops.SetPrefixMode() alongside r.prefixMode in metric binary/
  decimal set handlers (no deadlock: Operations.mu is separate from
  RPN.mu)
- Update MetricShow to display factor using current prefix mode
- Add SetPrefixMode to the Operator interface
- Add end-to-end tests verifying prefix mode affects conversion,
  arithmetic, and comparison results

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