M5a: cmux-linux GTK4 application scaffold (Sprint 1-3)

[Jesssullivan]

Summary

Complete scaffold for the cmux-linux GTK4 terminal multiplexer, implementing issues #77-83 across 3 sprints.

Sprint 1: Foundation

• Build cmux-linux GTK4 application linking to libghostty #77: cmux-linux Zig GTK4 app linking to libghostty via GHOSTTY_PLATFORM_LINUX
• Port configuration parser to Zig #82: JSON config parser with validation (command defs, recursive layouts, hex colors)
• Ghostty submodule switched to Jesssullivan/ghostty with linux Platform variant

Sprint 2: Core UI

• Implement CmuxWindow with AdwTabView tabs #78: AdwTabView + AdwTabBar for tabbed workspaces, tab_manager.zig
• Implement split pane management via SplitTree #79: Binary tree split pane model (GtkPaned), split_tree.zig
• Implement workspace model + sidebar #80: Workspace data model + sidebar (AdwNavigationSplitView + GtkListBox)

Sprint 3: Platform Integration

• Port socket command handler to Zig #81: Unix domain socket JSON-RPC server (XDG_RUNTIME_DIR, GLib integration)
• Port session persistence to Zig #83: Session snapshot/restore (autosave 8s, atomic writes, schema v1)

Files (11 new)

cmux-linux/
├── build.zig           # Links libghostty.a + GTK4 + libadwaita + deps
└── src/
    ├── main.zig        # GtkApplication + libghostty init
    ├── app.zig         # Runtime callbacks (action, clipboard, close)
    ├── c_api.zig       # @cImport bindings
    ├── config.zig      # JSON config parser + validation
    ├── session.zig     # Session snapshot + restore
    ├── sidebar.zig     # Workspace list sidebar
    ├── socket.zig      # Unix socket JSON-RPC server
    ├── split_tree.zig  # Binary tree split pane model
    ├── surface.zig     # GtkGLArea + ghostty surface
    ├── tab_manager.zig # AdwTabView workspace management
    └── workspace.zig   # Workspace data model

~2,000 lines of Zig across 11 source files.

Test plan

• cd ghostty && zig build -Dapp-runtime=none -Drenderer=opengl produces libghostty.a
• cd cmux-linux && zig build compiles and links on Linux
• App launches with GTK4 window, terminal renders
• New tab / close tab works
• Socket responds to ping

Closes #77, #78, #79, #80, #81, #82, #83

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