refactor: extract typing diagnostics instrumentation

[teamleaderleo]

Problem

AppDelegate.swift embedded three independent DEBUG instrumentation services: typing timing, main-run-loop stall detection, and main-thread turn profiling. Other app files already consume the timing API, so the delegate was also the accidental home for shared diagnostics.

Change

Move CmuxTypingTiming, CmuxMainRunLoopStallMonitor, and CmuxMainThreadTurnProfiler into dedicated Sources/Debug files. Names and APIs are unchanged; the application composition and existing instrumentation call sites continue to work. The new files are wired into the Xcode project.

This PR is stacked on #70.

Validation

• swiftc -D DEBUG -parse Sources/Debug/CmuxTypingTiming.swift Sources/Debug/CmuxMainRunLoopStallMonitor.swift Sources/Debug/CmuxMainThreadTurnProfiler.swift Sources/AppDelegate.swift
• git diff --check
• Confirmed no definitions remain in AppDelegate.swift.

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.