feat: Implement currency precision fallback

[younies]

This PR implements currency precision fallback, integrates pattern precision into CurrencyEssentials, and adds support for per-currency fallback (i.e., resolving precision based on the specific currency, such as 0 decimal places for JPY or 2 decimal places for USD, before falling back to a global default).

It is stacked on top of #8150.

Changelog

1. Add Pattern Precision into CurrencyEssentials:

   • Added fractions_vec, standard_index, and standard_next_to_alpha fields to CurrencyEssentials to store locale-specific pattern precision.
   • Updated the datagen provider for CurrencyEssentialsV1 to extract this precision from CLDR patterns at datagen time.
   • This optimizes Short and Narrow formatters by reducing the number of loaded data markers from 2 to 1.

2. Implement Currency Precision Fallback:

   • Updated CurrencyFormatter and CompactCurrencyFormatter to use CurrencyEssentials for locale-specific precision.
   • Implemented a 3-step fallback for resolving currency precision:
     1. Currency-specific override: Look up the currency in the global CurrencyFractionsV1 map (e.g., JPY = 0).
     2. Locale-specific pattern precision: If not found, use the precision of the selected pattern from CurrencyEssentials (only for Short/Narrow formats).
     3. Global default: Fall back to the global default in CurrencyFractionsV1 (2 decimals).
   • For Long format, since it doesn't use CurrencyEssentials, it bypasses step 2 and falls back directly to the global default (or currency override).

3. Support Per-Currency Fallback (ISO Code Fallback):

   • Updated CurrencyFormatter to make CurrencyExtendedDataV1 (display names) optional.
   • If the display name data for a currency is missing (e.g., for an unsupported or fake currency like XYZ), it now gracefully falls back to using the currency's ISO code as the display name instead of failing construction.

4. Tests and Data:

   • Added tests to verify the Long format fallback to ISO code (test_long_fallback_to_iso).
   • Updated existing tests to expect the correct rounded values (since precision is now active).
   • Regenerated baked data.

TAG=agy
CONV=4e806c61-d22c-4821-8a7f-6c8e0ebb5a29

[younies]

why Owned ?

[younies]

We need Cow::Owned here because value is a local variable.

In format_fixed_decimal, we apply precision which clones and modifies the input, creating a new local FixedDecimal (value). If we used Cow::Borrowed(&value.absolute), the returned FormattedDecimal would borrow from this local variable, which would result in a lifetime error since the local variable is dropped when the function returns.

By using Cow::Owned(value.absolute), we move the owned data into the Cow so it can be safely returned. Since UnsignedDecimal uses inline storage for typical digit counts, this Cow::Owned usually does not trigger heap allocations.

An alternative to avoid Cow::Owned would be to define a custom FormattedCurrency struct that owns the modified FixedDecimal and implements Writeable, but that would add more complexity for now.

[younies]

why do we allow this ?

[younies]

We allow IdentifierNotFound here to support fallback to the ISO code when the localized display name for a currency is missing.

According to UTS #35 (LDML Part 3: Numbers) Section 3.11.1:

If no displayName element is found at any level in the fallback chain, the system uses the currency code itself (e.g., "ZWD") as the display name.

If we didn't use allow_identifier_not_found()?, the formatter construction would fail completely for unsupported currencies. By allowing it, we can still format the number and gracefully fall back to the ISO code (e.g., 12,345.67 XYZ).

[younies]

add a comment to this function.

[younies]

Done. Added doc comments to both `compact_formatter.rs` and `formatter.rs`.