feat(examples): sokol + imgui smoke test example

[apotema]

Summary

Adds `examples/sokol_imgui/` — a minimal end-to-end test that exercises the full sokol + Dear ImGui + labelle-imgui bridge stack. If this example builds and runs with a clickable button, the entire chain is wired correctly:

• sokol backend with `with_sokol_imgui = true` (provided by the labelle-imgui sokol bridge that owns its own sokol-with-imgui dep)
• cimgui linked once into the final exe
• `sokol_imgui_bridge` artifact resolving `simgui_*` symbols
• generated `sokolEvent` forwarding sapp events to `imgui_bridge_handle_event` so simgui's IO state actually receives mouse / keyboard input

Diff against `examples/raylib`

The example is a near-copy of `examples/raylib` (same scenes, prefabs, components, gizmos, scene switcher, player movement) so the side-by-side diff between the two `project.labelle` files is the minimum surface area needed to add ImGui to a sokol project:

```zon
.backend = .sokol,
.gui = .{ .plugin = "imgui" },
.plugins = .{
.{ .name = "imgui", .repo = "local:../../../labelle-imgui" },
},
```

scripts/imgui_demo.zig

Opens a "Sokol + ImGui Bridge" window each frame and renders three widgets that each exercise a different piece of the IO path:

• label — proves the render side works
• button — proves `mouse_pos` + `mouse_button` events flow through
• slider — proves drag deltas accumulate via the same event channel

Dependencies

• Depends on fix(sokol): forward sapp events to the GUI bridge so ImGui IO works #38 (sokol event forwarding) — already merged. Without it the click counter and slider don't respond.
• Depends on labelle-toolkit/labelle-imgui main's existing sokol bridge (PRs Fix: rewrite relative .path deps when copying local libs (#129) #1 + feat: scan animations/ directory for .zon animation definitions #2).

Test plan

• `labelle build` against latest assembler main + labelle-imgui main
• Click counter increments on mouse click inside the ImGui button
• Slider drags and updates value
• Window can be dragged by its title bar

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Adds a new self-contained example project and assets without modifying engine/runtime code paths; risk is limited to example build/config integration with the local labelle-imgui plugin.

Overview
Adds a new examples/sokol_imgui demo project that runs on the Sokol backend and enables the imgui GUI plugin, intended as an end-to-end smoke test for the Sokol+Dear ImGui bridge.

The example includes minimal ECS components, prefabs, scenes (with include-based scene composition), and state-scoped scripts for menu/playing/paused flow, hot-reload (F5), and camera/player movement.

Adds scripts/imgui_demo.zig which renders an ImGui window each frame (label/button/slider + frame counter) to validate widget rendering and input event forwarding through the bridge.

^{Reviewed by Cursor Bugbot for commit a0a7d0b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[gemini-code-assist]

Code Review

This pull request introduces a new example project, sokol_imgui, which demonstrates the integration of Sokol and Dear ImGui within the engine. It includes a full suite of components, prefabs, scenes, and scripts to showcase features like player movement, state management, and UI rendering. However, several compilation issues were identified in the imgui_demo.zig script, specifically regarding a call to an undefined supportsWidgets function and type mismatches in catch expressions when formatting strings for GUI labels.

[gemini-code-assist]

The supportsWidgets() function is called on the Gui adapter, but it is not defined in either plugins/imgui/src/adapter.zig or gui/sokol-imgui/src/adapter.zig. This will cause a compilation error. If this is a required part of the GuiInterface contract, it should be implemented in the adapters.

[gemini-code-assist]

In Zig, the catch expression requires both branches to have the same type or coerce to a common type. std.fmt.bufPrintZ returns a sentinel-terminated slice [:0]u8, while the string literal "frame: ?" is a pointer to an array (*const [8:0]u8). This will result in a compilation error. Slicing the literal to [:0]const u8 resolves the type mismatch.

    Gui.label(std.fmt.bufPrintZ(&fps_buf, "frame: {d}", .{frame_counter}) catch "frame: ?"[0..:0]);

[gemini-code-assist]

Similar to the frame counter label, this catch block will fail to compile due to a type mismatch between the slice returned by bufPrintZ and the string literal. Slicing the literal to [:0]const u8 ensures compatibility with the expected [*:0]const u8 parameter of Gui.label.

    Gui.label(std.fmt.bufPrintZ(&click_buf, "clicks: {d}", .{click_count}) catch "clicks: ?"[0..:0]);