Export the plugin door only behind the dropped-in feature

[MattJackson]

The C entry symbol busbar_plugin_door is now exported only by the dropped-in cdylib build, never by an rlib a busbar build links as a compiled-in row (spec Part 2 #2, compiled-in = dropped-in, one table). export_door! sat unconditionally in this plugin crate, whose crate-type includes rlib; linking two such crates (or one beside its own cdylib) into one binary defines the symbol twice under lto = "fat".

• dropped-in = [] feature on the -plugin crate; export_door! is #[cfg(feature = "dropped-in")], the same shape as busbar-transport-ws.
• The crate names itself as a dev-dependency with dropped-in on, so cargo test still builds the cdylib the conformance test opens.
• Behaviour otherwise unchanged. The dropped-in artifact is now built with --features dropped-in (busbar plugin-release.yml/plugin-ci.yml build -p $CRATE and need to pass it).
• No new third-party dependency; Cargo.lock gains the self dev-dependency edge only.

[MattJackson]

Closed as superseded (ARCHITECT ruling 2026-10-03): #17 (lane-store-cold-boot) set the door exports for the busbar the pin boots. Note: gating export_door! behind dropped-in is not on dev. It comes back with the repin past busbar #433/#442.