Improve handling of placeholder currencies in unit normalization

[iwatt11]

TL;DR

Improved handling of placeholder currencies (e.g., "National currency") in unit labels and normalization.

What changed?

• Enhanced buildNormalizedUnit to properly handle placeholder currencies (marked as "UNKNOWN") by preserving the original label instead of attempting to replace it with a target currency
• Updated currency selection logic for unit labels:
  • When FX conversion occurs, use the target currency
  • When no conversion occurs, use the known effective currency
  • For placeholder currencies with no FX, preserve the original unit label
• Added special handling for per-capita and time-scale indicators with placeholder currencies
• Fixed currency component fields in explain metadata to properly reflect when a currency is unknown

How to test?

1. Test with indicators that use placeholder currencies like "National currency"
2. Verify that:
   • Original labels are preserved when no FX conversion occurs
   • Time scales are properly appended when appropriate
   • Per-capita calculations maintain the correct unit format
   • Currency components in explain metadata correctly reflect unknown currencies

Why make this change?

Previously, the system would attempt to replace placeholder currencies with target currencies even when no actual FX conversion was possible, resulting in misleading unit labels. This change ensures that when dealing with placeholder currencies like "National currency," the original label is preserved unless an actual currency conversion occurs, providing more accurate and meaningful unit representations.

[iwatt11]

• Improve handling of placeholder currencies in unit normalization #2 👈 (View in Graphite)
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