refactor: extract application construction and lifecycle

docs/APPLICATION.md

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+# Application construction
+
+`gods-eye-view/application` exports `createApplication`. Importing it does not
+create a viewer, discover configuration, start requests, or attach browser
+listeners. Construction is also inactive until the caller invokes `start()`.
+
+The caller supplies four constructors, in this order:
+
+| Constructor | Receives | Standalone implementation |
+| --- | --- | --- |
+| `createScene` | `signal`, `defer` | Viewer, attribution and initial map |
+| `createControls` | `scene`, `signal`, `defer` | Style manager and camera presentation |
+| `createData` | `scene`, `controls`, `signal`, `defer` | Layer catalog, registration and restoration |
+| `createTools` | `scene`, `controls`, `data`, `signal`, `defer` | Scenes, annotations, voice and page listeners |
+
+Each constructor returns its component object, or a promise for that object.
+Those objects are passed unchanged to later constructors. Configuration and
+service instances belong in the caller's closures; the application does not
+interpret provider names, environment variables, endpoints, or module paths.
+There is no module discovery or automatic import mechanism.
+
+```js
+import { createApplication } from 'gods-eye-view/application';
+import { createApplicationViewer } from 'gods-eye-view/application/viewer';
+
+const app = createApplication({
+  createScene({ defer }) {
+    const viewer = createApplicationViewer({ container, creditContainer });
+    defer(() => viewer.destroy());
+    return { viewer };
+  },
+  createControls,
+  createData,
+  createTools,
+});
+const unsubscribe = app.subscribe(({ status, phase }) => {
+  // Update the caller's startup presentation.
+});
+await app.start();
+// When the application is no longer needed:
+unsubscribe();
+await app.destroy();
+```
+
+The example's containers and three remaining constructors are supplied by the
+consumer. The viewer helper preserves the standalone viewer's render settings;
+it neither selects map sources nor reads keys. Supply a visible credit container
+and retain all attribution required by the chosen sources. Cesium is external to
+this export: the consuming build must provide the same instance used elsewhere
+in its application.
+
+## Lifecycle and ownership
+
+Register `defer(cleanup)` immediately after acquiring each resource, before any
+`await`. Cleanup registration remains open until that constructor settles. Each
+callback may return a promise. Within a phase, callbacks run in reverse order.
+Across phases, teardown runs **tools, controls, data, scene**: controls must cancel
+restoration while their data manager and viewer are still alive.
+
+`start()` returns the same promise on repeated calls. `destroy()` is terminal and
+also returns the same promise on repeated calls. It aborts the shared signal
+immediately, waits for an in-flight constructor to settle, and runs all registered
+cleanup callbacks. Constructors must forward the signal to cancellable operations
+and check it after awaits. If a provider cannot cancel construction, register
+cleanup for its late result before returning. Destruction waits for that result;
+it does not pretend a pending resource has already been released.
+
+A constructor failure triggers the same cleanup before rejecting startup.
+Cleanup failures do not stop the remaining callbacks; they are reported through
+an `AggregateError`. Constructors own cleanup for resources created internally,
+including anything allocated before their own construction throws.
+
+`getState()` returns a frozen `{ status, phase }` snapshot. Status is `created`,
+`starting`, `ready`, `destroying`, `destroyed`, or `failed`. `phase` identifies the
+constructor during startup. `subscribe(listener)` immediately reports the current
+snapshot and returns an unsubscribe function. Install observers before `start()`
+to receive every startup phase. Observer failures cannot interrupt startup or
+teardown. `getComponents()` returns a frozen, shallow snapshot of the currently
+constructed component objects; the component instances themselves remain mutable.
+
+`ready` means all constructors have returned. Background feed activity and the
+standalone share-restoration result keep their existing separate contracts.
+
+## Standalone wiring
+
+`src/main.js` reads the existing browser configuration and starts
+`src/editions/local/application.js`. That module selects the four implementations
+in its directory. Scene setup, controls, layer registration, tools and loading
+chrome have separate owners. The existing `window.__godsEyeView` debugging shape
+is preserved while the app is running.
+
+The standalone controls and layer modules still contain page-scoped state.
+Only one standalone application may be constructed per page. Shutdown releases
+its runtime resources and debugging handles; reload the page to start again.
+It is not an embeddable, removable HTML shell or a multiple-viewer implementation.
+Consumers of the small application export supply their own component ownership;
+the lifecycle controller itself has no shared instance state.
+
+Later component extractions can replace a constructor's internals without
+changing the startup contract or moving standalone imports into the package.
+Run the unit suite, package boundary gate, build, tracking and first-run browser
+checks when changing this wiring.