fix(browser): own transport cleanup through finalization

Own CDP transports from admission through observed close, retain failed
owners for retry, and keep active peers separate from pending cleanup.
Preserve primary errors and completed facts through final cancellation.
Forward passive Electron CLI cancellation and dispose browser, runtime and
scenario owners from CLI and MCP shutdown.

Refs #4255, #4256, #4257

src/application/javascript/ElectronObservationPort.ts

@@ -13,6 +13,8 @@ import type { Result } from "../../domain/result.js";
 
 /** Provider-neutral boundary for root-confined Electron file-page observation. */
 export interface ElectronObservationPort {
+  /** Release provider-owned transports during command or server shutdown. */
+  close?(): Promise<void>;
   identity(): ProviderIdentity;
   listTargets(
     input: ListElectronTargetsInput,