Handle fill value edge cases (NaN, Inf, variable-length strings)

[jhamman]

Summary

The Zarr V3 spec supports fill values that include special floating-point values (NaN, Infinity, -Infinity) and variable-length data types (e.g., variable-length UTF-8 strings). The current implementation may not handle all of these correctly.

Specific cases to address

Special float fill values

• NaN — Zarr V3 encodes as the string "NaN" in JSON metadata
• Infinity / -Infinity — encoded as "Infinity" / "-Infinity"
• These need special parsing since JSON doesn't natively support these values

Variable-length strings

• Zarr V3 has a string data type with variable-length UTF-8 encoding
• Fill value is typically "" (empty string)
• The current Julia bindings support 14 numeric types but may not handle string arrays

Acceptance criteria

• Arrays with NaN fill value can be created and read back correctly
• Arrays with Infinity/-Infinity fill values work correctly
• Add tests for special fill value round-trips
• Document any data type limitations

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