feat(shell): tabbed central panel with pinned home for embedding hosts

[samuelstroschein]

What

An opt-in embedding API for hosts that need browser-style tabs in the central panel (LixRay's home flow), while keeping the current single-document slot as the default (FlashType is unaffected — no changes needed there).

createAtelier({
  lix,
  extensions: [homeExtension, dirExtension],
  centralPanel: {
    tabs: true,                          // central tab strip + multiple content views
    home: { extensionId: "lixray_home" } // pinned, non-closable first tab
  },
});

Full write-up in docs/embedding.md.

The tab model (browser-like)

1. The configured home is pinned first, never closes, never navigates; it compacts to its icon when inactive.
2. documents.open(path) activates an existing tab for that path, otherwise navigates the active content tab in place (label follows the location). If home is active, a content tab is appended.
3. documents.open(path, { newTab: true }) (hosts map ⌘-click) and the tab-strip ＋ append new tabs.
4. Closing a tab activates its neighbor; the last content tab closes back to home.

API surface

• AtelierOptions.centralPanel (tabs, home)
• AtelierDocumentOpenOptions.newTab
• instance.views.open(extensionId, { state, instanceKey, newTab, panel }) — places host extension views (e.g. folder views) as central content tabs; also on AtelierExtensionRuntime.views
• ExtensionManifest.placement / .hidden — declare central-capable, menu-hidden views
• central_view_activated event — fired for every active-central-view change so routing hosts can mirror the active tab into the URL

Compatibility

All additive. Without centralPanel the shell behaves exactly as before (verified by the existing 838-test suite); new behavior is covered by src/shell/central-tabs.test.tsx.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	atelier-preview	5de946d	Commit Preview URL Branch Preview URL	Jul 21 2026, 09:34 PM

[samuelstroschein]

Rebased onto main (post checkpoint-removal) and applied the review round:

• centralPanel is now a mode-discriminated union: { mode: "document" } | { mode: "tabs", home? } — a pinned home can no longer be configured without tabs.
• The tab-model rules moved into a CentralSlotBehavior strategy object (src/shell/central-slot-behavior.ts) with one implementation per mode, replacing the boolean-branching policy threading; the behavior parameter is required at every call site.
• views.open instanceKey → instanceId (same value the host gets back on central_view_activated), reserved-id guard for central-home, and completion now waits for activation.
• Fixes from adversarial review: Files view no longer vanishes under landing mode with a pinned home (forced to sidebar); document_closed.nextFilePath reports the tab that actually activates; placement is enforced for side panels (menus, drag&drop, programmatic opens); reopening an existing tab merges state instead of resetting it; central_view_activated fires in both modes; extension_opened no longer fires on no-op resyncs; isPinned is validated on persisted state.

838 tests green, lint clean.

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