perf: lazy-load editor runtime

Issue: #337
User-Visible: yes

src/editor-runtime-loader.ts

@@ -0,0 +1,72 @@
+export type EditorRuntimeLoaderState = 'idle' | 'loading' | 'ready' | 'failed';
+
+export interface EditorRuntimeModule<Runtime> {
+  readonly fingerprint: string;
+  create(): Runtime;
+}
+
+export interface EditorRuntimeLoaderOptions<Runtime> {
+  readonly expectedFingerprint: string;
+  readonly load: (attempt: 0 | 1) => Promise<EditorRuntimeModule<Runtime>>;
+  readonly install: (runtime: Runtime) => void;
+  readonly stateChanged?: (state: EditorRuntimeLoaderState) => void;
+  readonly failed?: (error: unknown) => void;
+}
+
+/**
+ * One atomic lazy-runtime boundary shared by all editor entry points.
+ *
+ * A failed module is retried exactly once. Construction happens before
+ * `install`, so a parse, fingerprint or constructor failure cannot leave a
+ * half-installed editor attached to the View card.
+ */
+export class EditorRuntimeLoader<Runtime> {
+  private _state: EditorRuntimeLoaderState = 'idle';
+  private _inFlight: Promise<boolean> | null = null;
+
+  public constructor(private readonly options: EditorRuntimeLoaderOptions<Runtime>) {}
+
+  public get state(): EditorRuntimeLoaderState {
+    return this._state;
+  }
+
+  public ensure(): Promise<boolean> {
+    if (this._state === 'ready') return Promise.resolve(true);
+    if (this._state === 'failed') return Promise.resolve(false);
+    if (this._inFlight) return this._inFlight;
+    this._setState('loading');
+    this._inFlight = this._loadWithRetry().finally(() => {
+      this._inFlight = null;
+    });
+    return this._inFlight;
+  }
+
+  private async _loadWithRetry(): Promise<boolean> {
+    let lastError: unknown = new Error('Editor runtime did not load');
+    for (const attempt of [0, 1] as const) {
+      try {
+        const module = await this.options.load(attempt);
+        if (module.fingerprint !== this.options.expectedFingerprint) {
+          throw new Error(
+            `Editor runtime fingerprint mismatch: expected ${this.options.expectedFingerprint}, got ${module.fingerprint}`,
+          );
+        }
+        const runtime = module.create();
+        this.options.install(runtime);
+        this._setState('ready');
+        return true;
+      } catch (error: unknown) {
+        lastError = error;
+      }
+    }
+    this._setState('failed');
+    this.options.failed?.(lastError);
+    return false;
+  }
+
+  private _setState(state: EditorRuntimeLoaderState): void {
+    if (state === this._state) return;
+    this._state = state;
+    this.options.stateChanged?.(state);
+  }
+}