Serialize engine and shell transitions through the scene host

Move engine request ordering and the single rollback decision into the scene
host. Run handoff, renderer release, candidate acquisition, attachment,
restoration and settling in one transaction. Pass that transaction to nested
parts so unrelated calls wait without putting nested work behind itself.
Keep the engine-switch entry point as a delegating adapter.

Route direct shell attachment and detachment through the same host and remove
the shell queue. Serialize late part registration with attachment and release.
Keep failed-attachment cleanup and renderer release inside the transaction,
including failed restoration. Replacement applications share the transition
owner with fresh parts, and retired applications release their part registry.
A shell used alone creates its own scene host.

Preserve Cesium's picker and history ordering, single recovery, retained layer
session, cockpit restoration, hidden-tab settling and parked-renderer policy.
Navara keeps the same behavior. No user-visible change is intended on either
engine. Keep the existing behavioral assertions and cover concurrent operations,
failed cleanup followed by recovery, retired transaction handles, replacement
startup failure and parked expiry during a switch.

All eleven coupling counts remain unchanged; none fell or grew. Renderer lines
remain 0 and 189, external Cesium import files 3, viewer lines 891, Navara kind
checks 1, engine-sniff calls 0, extension reads 3, ground passes 1, SDK loads 3,
screen-handler files 4 and engine-name lines 32.

Checked on the keyless production build: the engine, Director and layer
handoff switches, app-once, and the rapid, hidden-tab, context-loss,
parked-release, failure-matrix and soak scenarios pass.

scripts/package-boundaries.json

@@ -1031,6 +1031,8 @@
       "src/ui/applicationShell.js",
       "src/app/layerSession.js",
       "src/ui/shellScene.js",
+      "src/app/sceneHost.js",
+      "src/app/engineChoice.js",
       "src/ui/localePicker.js",
       "src/ui/cyberSonarControls.js",
       "src/ui/locationNavigation.js",
@@ -2940,6 +2942,8 @@
       "src/ui/applicationShell.js",
       "src/app/layerSession.js",
       "src/ui/shellScene.js",
+      "src/app/sceneHost.js",
+      "src/app/engineChoice.js",
       "src/ui/mobileShell.js",
       "src/ui/localePicker.js",
       "src/ui/cyberSonarControls.js",