Add WSL version capabilities

[mveril]

Summary

• Adds explicit WSL version capability modeling so plugin code can reason about features by capability instead of raw version checks.
• Exposes capability support helpers on WSLVersion and lets the proc macro declare one or more required capabilities.

Changes

• Added WSLVersion and WSLVersionCapability support APIs, including WSLVersion::supports and WSLVersion::capabilities.
• Updated plugin registration utilities to select the highest required capability version when creating plugins.
• Extended #[wsl_plugin_v1(...)] parsing so required capabilities can be combined with |.
• Added macro coverage for capability requirements.

Components Touched

• Public API
• Proc macro
• Runtime internals
• Unsafe code
• Bug fix
• Documentation
• Examples
• CI / release workflow
• AGENTS.md instructions

Validation

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features
• cargo fmt --all -- --check
• Relevant manual testing completed

WSL / Windows Notes

• This affects plugin creation metadata by deriving the required WSL version from declared capabilities.
• No manual Windows plugin loading, signing, registration, or WSL service restart validation was performed.

Checklist

• Tests added or updated when relevant
• Documentation updated when relevant
• Examples updated when relevant
• No unrelated changes included