refactor: route test window mutations through registry

[teamleaderleo]

Problem

After extracting MainWindowRegistry, a few test support paths still mutated AppDelegate.mainWindowContexts directly, bypassing the registry's identity invariants.

Change

Add an ObjectIdentifier insertion API and route the remaining test-only context mutations through MainWindowRegistry. Production routing, notifications, recovery, and lifecycle side effects remain in AppDelegate. No direct mainWindowContexts[...] = assignments remain in Sources or cmuxTests.

This PR is stacked on #76.

Validation

• swiftc -D DEBUG -parse Sources/MainWindowRegistry.swift Sources/CmuxMainWindowContext.swift Sources/AppDelegate.swift cmuxTests/AppDelegateMainWindowTestingSupport.swift cmuxTests/WorkspaceRecoveryTests.swift
• git diff --check

A full xcodebuild remains blocked by the fork worktree's missing vendor/bonsplit/Package.swift dependency.

[teamleaderleo]

Consolidated into the coherent upstream slices #78 (UI-test infrastructure), #79 (session lifecycle), #80 (window ownership), and #82 (terminal diagnostics). The original branch remains available for history.