🤖 [displaynames] Refactor fallback implementation to reduce diff size and improve Rustiness

- Reuse ScriptDisplayNameOwned, RegionDisplayNameOwned, and VariantDisplayNameOwned constructors in try_new_unstable to eliminate duplicate loading boilerplate.
- Replace PayloadOrFallback enum with a type alias to DataPayloadOr<M, TinyAsciiStr<8>>, reusing robust existing implementations and eliminating custom Debug impl.
- Use precise DataPayloadOr<LocaleNamesLanguageMediumV1, TinyAsciiStr<4>> for language_payload, avoiding unreachable Absent case.
- Implement elegant Rusty iterator chain for variant loading.
- Restore all original comments in language.rs.
- Preserve original test_concatenate structure in tests.rs, running both strict and lenient checks in the same loop, and adding test_fallback_parts.

Co-authored-by: Gemini <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: sffc