feat(prelude): support shared native mobile code

Summary

• Adds shared mobile code target kinds for Swift into Android native libraries and Kotlin/Native into Apple frameworks, so cross-platform source sharing is modeled directly in the prelude.
• Extends Android and Apple consumers to use generic native provider contracts, which keeps platform-specific behavior in Starlark target kinds instead of adding core graph special cases.
• Extends rust_library so Rust static libraries can feed Apple link targets and Rust Android dynamic libraries can be packaged into APKs.
• Adds starter examples, a composed native-mobile-shared-code-e2e app graph, schema coverage, focused prelude tests, and reference docs for the new contracts.

Testing

• mise exec -- cargo test -p once-frontend
• mise exec -- cargo fmt --all -- --check
• git diff --check
• Manual check: scanned touched rule, test, and docs files for prohibited punctuation.

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