Report the Linux display backend and document the Wayland boundary

[OthmaneBlial]

Context

RustFrame currently supports Linux through the GTK/WebKit path and the public roadmap calls out an X11-only boundary. Doctor output should identify the active display session and give precise guidance instead of allowing a broad cross-platform claim.

Affected area

• rustframe doctor and doctor --json
• Linux host diagnostics
• docs/platform-support.md and troubleshooting
• CLI fixture tests

Acceptance criteria

• Doctor distinguishes X11, native Wayland, XWayland, headless, and unknown sessions from deterministic fixture inputs.
• JSON output adds a documented stable field without exposing unrelated environment data.
• Human output explains which modes are supported, experimental, or unsupported and names remediation steps.
• Tests run without requiring a live Linux desktop session.
• Public platform documentation matches the diagnostic result.

Test command

cargo test -p rustframe-cli doctor
cargo clippy -p rustframe-cli --all-targets -- -D warnings
node scripts/check_public_contracts.mjs

Non-goals

• Claiming native Wayland support before a real packaged-app matrix exists.
• Replacing GTK or WebKitGTK.
• Adding telemetry.