feat(workspaces): sync the home tile layout per device orientation

The Win8-Start home grid's arrangement now syncs from the backend's
home-tile-layout plugin: ONE shared document per device orientation
(wide = landscape/desktop, tall = portrait/mobile) — every landscape
device renders the same layout, every portrait device the same other
one. localStorage persistence is retired (no migration — devices start
from the shared document; the first arrangement creates it).

- adapter/tile-layout-http.ts: the injected HTTP edge (GET/PUT/DELETE
  /home/tile-layout/:mode), the features/layout adapter's pattern
- tile-layout-store: server-backed, keyed by geometry mode — boot +
  orientation-change probes (stale in-flight responses never land),
  locally-applied edits + debounced full-document PUT with rollback to
  the last server document, WS push filtered to the active orientation,
  quiet ephemeral degrade when the backend is unreachable (no error UI),
  sync gated while a probe is in flight
- WorkspacesHome locks the store's orientation to the resolved grid
  geometry (the landscape/portrait split IS the device-type split)
- app store: the tile store lives beside the layout store; the WS
  dispatch feeds 'tile-layout' broadcasts into it; 'tiles' prop from
  App.svelte (the page entry owns the httpBase)
- ws adapter: the 'tile-layout' server message (parse guard) + socket
  handler; wire conformance case

1 files changed, 2 insertions, 0 deletions

diff --git a/src/core/wire/conformance.ts b/src/core/wire/conformance.ts
index 99afc6d..b358641 100644
--- a/src/core/wire/conformance.ts
+++ b/src/core/wire/conformance.ts
@@ -95,6 +95,8 @@ export function assertWsServerMessageExhaustive(msg: WsServerMessage): string {
       return "extensions-changed";
     case "layout":
       return "layout";
+    case "tile-layout":
+      return "tile-layout";
     default:
       return msg satisfies never;
   }