rxe/editor: the editor as a module, apps/editor as its main

[Force67]

Phase 6c of docs/STRUCTURE.md.

• rx::editor (rxe/editor): the editor app code (Editor, its UI, input, terrain and placement tools, editor.ugui) plus selection and undo from rxe/edit, which is gone (rx::edit becomes rx::editor). apps/editor is now just main.cc. A game's editor is the same few lines linking its own modules next to rx::editor, and the install package exports rx::editor.
• The module takes the rx_editor target name; the executable target is rx_editor_app with OUTPUT_NAME rx_editor, so the binary users run keeps its name (build/linux/apps/editor/rx_editor).
• terrain moves from plugins/ into rxe/. Terrain editing is built into Editor (a mode with its own undo hooks and UI), and an engine module cannot depend on a plugin. It can go back to plugins/ with its editor mode once the editor has an extension point.
• Lint: only apps and the editor may depend on app. The editor is the one engine module that is itself an application; games extend it. Everything under rxe/editor/ is editor-only for other libraries. The editor's own action enum moves from flat rx:: into rx::editor; the namespace lint caught it.
• world, rxworld and the shell no longer link edit; they only needed what moved into scene in authoring becomes devtools, scene takes reflection and hierarchy #81.
• Drops the editor's per-DSO InitOptionsFromEnv workaround (obsolete since Shared build links and runs: exports, one option registry, per-DSO volk, linux-dev #84).

Verified:

• static and shared: build, ctest (same 155 tests, 155 pass) in both; lint 0 violations
• swrun rx_editor starts, mounts rxe://, reports 0 platform-config problems, and is still running at a 15 s timeout