refactor: extract UI test diagnostics writer

[teamleaderleo]

Problem

AppDelegate owned UI-test diagnostics manifest writing, window/display state serialization, render stats, and portal stats alongside application lifecycle code.

Change

Extract that responsibility into UITestDiagnosticsWriter under Sources/Debug/UITests. AppDelegate now supplies only narrow socket and focused-render closures and forwards diagnostics writes to the writer. The new source is wired into the Xcode project.

Validation

• swiftc -D DEBUG -parse Sources/Debug/UITests/UITestDiagnosticsWriter.swift Sources/AppDelegate.swift
• git diff --check

A full xcodebuild was blocked before compilation because this fork worktree is missing vendor/bonsplit/Package.swift (uninitialized 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.