Add a native-only desktop host

[WhiteHades]

Summary

• add host = "native" as a strict canvas/native-view desktop host mode
• omit WebKit, WebKitGTK, WebView2, CEF, frontend staging, and built-in JavaScript command dispatch from native-host packages
• reject conflicting frontend, WebView, bridge, Chromium, and CEF declarations at validation, build, compile, and package boundaries
• add a native-only fixture with null/Linux runtime checks, Linux ELF audits, Windows PE cross-audits, package audits, and host-specific macOS checks

Verification

• mise x pnpm@10.23.0 -- scripts/gate.sh full
• mise x pnpm@10.23.0 -- scripts/gate.sh fast on current upstream main
• Linux native binary/package imports contain no browser dependencies
• Windows GNU cross-compile and PE import audits pass
• macOS compile/link/runtime audits remain host-gated and run only on macOS

This is the generic SDK prerequisite tracked by WhiteHades/melearner#12.

[vercel]

@WhiteHades is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[ctate]

@WhiteHades Great call - I've started a series of PRs related to this, starting with #106, and adding you as co-author for these

Thanks for pointing this out and taking the time to open this PR