feat: Implement tmux control mode multi-pane management

[eyelock]

Summary

Adds native multi-pane terminal rendering via tmux control mode (-CC), with a three-backend architecture giving users a clear choice between Direct, TMUX Attach, and TMUX Control sessions.

Backends

Backend	Persistence	Pane management
Direct	Ephemeral — ends with app	No
TMUX Attach	Persistent across restarts	No
TMUX Control	Persistent across restarts	Yes

Pane management

• Real-time multi-pane layout via TmuxControlModeParser and list-panes
• Single NSViewRepresentable owns all pane views and a separator overlay, guaranteeing correct AppKit z-order without SwiftUI ZStack ambiguity
• Pane frames absorb the tmux 1-unit border gap so tiles are edge-to-edge with no dark gap
• 1pt accent-colour focus rect wall-to-wall with window chrome; single-pane sessions have no border
• Zoom detection hides sibling panes and suppresses phantom separator lines
• Directional navigation queries list-panes after select-pane to keep activePaneId in sync (select-pane does not trigger %layout-change)

Logging

• Add TermQLogger.swift: structured logging to Apple Unified Logging with categories tmux, pane, session, focus, io, ui
• Replace all print()/NSLog() calls across utilities and services
• Gate TermQLogger.io behind fileLoggingEnabled — terminal output is user data and must not reach Unified Logging unconditionally

Testing

• make check passes
• Split horizontal / vertical — new panes render correctly
• Click-to-focus — border and keyboard focus update
• Directional navigation (arrow buttons) — active pane changes
• Zoom / un-zoom — sibling panes hide/show, no phantom separator lines
• Close pane — layout contracts correctly
• TMUX Attach mode unaffected — single pane, no controls toolbar
• Ctrl+T respects defaultBackend setting

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