fix(web): report real GPU backend; unblock the WebGL2 path

[proggeramlug]

What

The web build asks wgpu for BROWSER_WEBGPU | GL and compiles wgpu's webgl feature — but the GL path could never run. Three separate bugs on it, each fatal on its own:

1. No display handle on the instance. wgpu-core passes raw_display_handle: None for a canvas surface target and then errors MissingDisplayHandle unless the instance carries one (wgpu-core 29 instance.rs:254). Surface creation died immediately. The WebGPU backend never reads it.
2. Limits::default() is unsatisfiable on GL. WebGL2 has no compute, so max_compute_workgroups_per_dimension is 0 against a default request of 65535 → request_device fails outright.
3. Unconditional COPY_SRC on the swapchain. A WebGL2 surface doesn't advertise it, so Surface::configure rejects the config.

Also: log the backend and adapter wgpu actually selected, instead of hardcoding "(WebGPU)".

Why it matters even though WebGL2 still can't render

A user reported the web build dying with No WebGPU/WebGL adapter found on macOS Sequoia. The root cause is that having navigator.gpu is not the same as having an adapter — Chrome ships the API on by default but hands back no adapter when graphics acceleration is off, the GPU is blocklisted, or it's a VM/remote session. wgpu picks its backend once, in Instance::new, purely on whether navigator.gpu is defined (wgpu-29 api/instance.rs:74), so it commits to WebGPU and never looks at the GL backend we compiled in.

These three fixes are the prerequisites for the GL path ever working. They are not sufficient: Renderer::new_impl eagerly creates 14 compute pipelines plus storage buffers and a 64KB joint uniform, and WebGL2 (GLES 3.0) has no compute shaders or storage buffers at all — wgpu only reports COMPUTE_SHADERS at GLES 3.1+. A real fallback needs a downlevel / 2D-only renderer init path (fits the existing setDirect2DMode idea). Filing this separately.

Risk

Inert on WebGPU. The limits change is explicitly scoped to Backend::Gl, so the WebGPU path requests exactly the limits it always has; the COPY_SRC mask is a no-op wherever the surface advertises the usage (i.e. everywhere WebGPU runs today).

Verification

Built for wasm32-unknown-unknown against current main. Drove the resulting build in a real headless Chrome:

• WebGPU path: boots, Bloom engine initialized (BrowserWebGpu on …), game renders and plays into level 1.
• GL path (forced by hiding navigator.gpu): now gets past instance/surface/device/swapchain creation and reaches renderer init, where it hits the compute-pipeline wall described above — i.e. exactly as far as these fixes are meant to take it.

Summary by CodeRabbit

• Bug Fixes
  • Improved WebGL2 compatibility during graphics initialization.
  • Prevented startup failures caused by unsupported device limits or texture operations.
  • Added more accurate initialization diagnostics, including the active graphics backend and adapter.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: bbb47cc3-50dd-444c-b2af-cbea9e774c8c

📥 Commits

Reviewing files that changed from the base of the PR and between 1d75f0c and 42cfe93.

📒 Files selected for processing (1)

• native/web/src/lib.rs

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📝 Walkthrough

Walkthrough

Changes

Web initialization compatibility

Layer / File(s)	Summary
Display handle and instance setup native/web/src/lib.rs	Adds WebDisplay with HasDisplayHandle and uses it when constructing the wgpu instance.
Backend-adaptive device and surface configuration native/web/src/lib.rs	Uses adapter limits for the GL backend, masks surface usage by advertised capabilities, and logs the selected backend and adapter name.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main change: fixing backend reporting and unblocking the WebGL2 path.
Description check	✅ Passed	The description covers summary, motivation, risks, and verification, though its headings differ from the template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/web-webgpu-adapter-and-gl-prereqs

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