editor: don't put an empty plugin manager in an empty rail

[Snoopy147]

What

A visitor who drops into the open lobby (/play/<id>), plays, then presses Esc lands on the read-only canvas — with a bare Plugins panel (heading, one line of copy, a "Create a Pascal plugin" link) eating ~40% of the window over the world they came to play in.

The lobby mounts <Editor isVersionPreviewMode layoutVersion="v2"> under a read-only scene lease and registers no host panels, so the plugin manager — the one tab the editor contributes itself — was alone in the rail. LeftColumn opens on its first tab, so the empty manager opened by default.

How

Two pure functions in lib/plugin-panels.ts:

• managedPluginIds(panels) — the distinct plugins behind the registered panels (one plugin may contribute several panels; the manager lists plugins).
• showsPluginManager({managedPluginCount, readOnly, workspaceMode}) — the tab earns its slot when there is a plugin to manage, or when the scene is writable and "Create a Pascal plugin" is still worth offering.

useHostPanels now consults it. The only case dropped is read-only and nothing registered, which is exactly the lobby — and with zero tabs both EditorLayoutV2 and EditorLayoutMobile already skip the sidebar entirely (sidebarTabs.length > 0), so the lobby becomes canvas and nothing else.

What does not change

• Writable edit workspace: identical for every plugin count, fenced by a regression test.
• Read-only editor with plugins registered: keeps the tab. Browsing what a project uses is a read; the Install button was already disabled={readOnly}.
• Studio workspace: unchanged (already excluded).

Verification

• bun test src in packages/editor — 807 pass / 0 fail, 10357 assertions (8 of them new, in plugin-panels.test.ts).
• bun run check-types at the repo root — 11/11 tasks successful.
• biome check — 1953 files, clean.

🤖 Generated with Claude Code