refactor: extract main window registry index

[teamleaderleo]

Problem

AppDelegate directly owned the main-window identity dictionary and repeated insert, reindex, conflict, and removal logic throughout lifecycle code.

Change

Extract the index invariants into MainWindowRegistry. It owns direct lookup, insertion, removal of all references, and reindex conflict behavior. AppDelegate retains a read-only mainWindowContexts compatibility view and all routing, notification, close, and recovery side effects.

This PR is stacked on #74.

Validation

• swiftc -D DEBUG -parse Sources/MainWindowRegistry.swift Sources/AppDelegate.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.