Test preview against pinned Blitz native renderer

[nicoburns]

Summary

Route the preview crate's native feature directly through dioxus-native pinned to Blitz revision 922938c6c8db76bc59ebbf0e8db89574354a3aff, while retaining the normal dioxus dependency for web, desktop, and server builds.

The updated Blitz graph requires Rust 1.89, so the preview MSRV and fixed CI toolchains move together. The README documents the platform-specific native preview command.

Testing

• cargo +1.89.0 fmt --all -- --check
• cargo +1.89.0 check -p preview --features native
• cargo +1.89.0 build -p preview --features native
• cargo +1.89.0 clippy --workspace --examples --tests --all-features --all-targets -- -D warnings
• cargo +1.89.0 test --workspace

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/d7586bc1d1a340e1b5f320911a9e21a6
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/d7586bc1d1a340e1b5f320911a9e21a6?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

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[github-actions]

Preview available at https://dioxuslabs.github.io/dioxus-components/pr-preview/pr-296/