feat(macOS): add optional LINUX DO guest browsing plugin

[1lck]

Summary

• add an official LINUX DO support plugin for macOS and keep it disabled by default so users opt in from plugin settings
• provide a compact read-only guest browser in the right tool window with hover reveal, restricted site navigation, and blocked authentication/write routes
• retain the WebKit page across short panel dismissals, preserve navigation and scroll state, and release the WebView after 10 minutes of inactivity
• add the reusable Rust Core Discourse protocol, deterministic contracts, fixtures, and macOS composition boundaries for future platform reuse
• prevent AppKit-backed controls and vector activity-bar icons from being rasterized into unavailable placeholders or inconsistent sizes

Verification

• ./scripts/test-macos.sh — 422 tests in 47 suites passed
• ./scripts/verify-service-boundaries.sh
• ./scripts/verify-shared-contracts.sh
• ./scripts/verify-rust-core-comments.sh
• cargo test --manifest-path rust/Cargo.toml -p lithe-core — 180 tests passed
• cargo fmt --manifest-path rust/Cargo.toml --all -- --check
• git diff --check

Current product behavior

The visible plugin experience is anonymous and read-only because LINUX DO currently does not issue Discourse User API keys and its anonymous JSON endpoints are protected by Cloudflare. The protocol implementation remains behind the shared Rust boundary for future reuse; the macOS browser blocks login, registration, reply, and posting routes.