feat(xlang): use xlang as default mode for all languages

[chaokunyang]

Why?

Fory 1.0 should make xlang the default path so new users get the cross-language wire format and compatible schema evolution without extra configuration. Native modes remain available for same-language workloads that need language-specific object features or native-only wire formats.

What does this PR do?

• Defaults Java, Python, Go, and Rust runtime construction to xlang mode, with explicit native-mode opt-outs for same-language serialization paths.
• Keeps Swift on the xlang-only path by removing the public xlang configuration switch and always writing/expecting the xlang header.
• Updates tests across Java, Python, Go, Rust, Swift, C++, Scala, and integration fixtures so native-mode expectations opt out explicitly.
• Refreshes the xlang, Java native-mode, schema metadata, configuration, and type-registration docs across runtimes for the new default.
• Adds or renames schema metadata guide pages and removes stale migration/native-mode docs that no longer match the 1.0 model.

Related issues

#1017

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

Not run; this PR changes defaults, tests, and documentation rather than benchmark logic.