App and Surface own the runtime wiring.

`main` creates core app state, initializes `apprt.App`, starts the optional quit timer, and runs the GUI event loop. Runtime-specific surface creation happens after this handoff.

const app: *App = try App.create(alloc);
var app_runtime: apprt.App = undefined;
try app_runtime.init(app, .{});
try app_runtime.run();

`Surface.init` first converts user config into derived config, initializes the runtime surface, computes DPI-aware font sizing, references the shared font grid, calculates renderer sizing, and initializes the renderer implementation with a surface mailbox.

const font_grid_key, const font_grid = try app.font_grid_set.ref(
    &derived_config.font,
    font_size,
);

var renderer_impl = try Renderer.init(alloc, .{
    .font_grid = font_grid,
    .size = size,
    .surface_mailbox = .{ .surface = self, .app = app_mailbox },
});

After renderer thread and IO thread managers exist, the surface creates the exec backend, SPSC IO mailbox, and `Termio`. The renderer state stores a pointer to `self.io.terminal`; thread startup happens only after Termio has been initialized and the first resize/finalization work has completed.

.renderer_state = .{
    .mutex = mutex,
    .terminal = &self.io.terminal,
},
.io = undefined,
.io_thread = io_thread,

self.renderer_thr = try std.Thread.spawn(.{}, rendererpkg.Thread.threadMain, .{&self.renderer_thread});
self.io_thr = try std.Thread.spawn(.{}, termio.Thread.threadMain, .{ &self.io_thread, &self.io });