O4: positron-wgpu — GPU renderer (outlier B, closes the outlier pair)

[joelteply]

What

positron-wgpu — the GPU reference Renderer for positron, outlier B in the contract's outlier-validation.

A Renderer here has type Output = Frame: GPU-bound geometry (colored quads for a vertex buffer), not a CPU text tree. counter-cli's String and positron-ratatui's Paragraph are both CPU text (a weak outlier pair). GPU quads are maximally different. With Renderer<S> fitting a String, terminal cells, and GPU geometry without forcing, the contract provably carries no hidden DOM/text assumption — so the middle (mobile, DOM) is guaranteed. That is "web ≠ DOM" made real: one Rust wgpu renderer runs native (Metal/Vulkan/DX12) + web (WebGPU/WASM) from one source.

Shape (same seam as outlier A)

• frame.rs — pure data (Frame/Primitive/Rect/Rgba/RgbaFrame), no wgpu. The render(state) -> Frame projection is unit-tested headlessly. RgbaFrame is deliberately continuum's avatar-frame shape (width/height/tight RGBA8) for the O5 reconcile.
• gpu.rs — the only file touching wgpu. Gpu::rasterize renders a Frame offscreen (wgpu 22, Rgba8Unorm, padded copy_texture_to_buffer readback). Missing GPU → loud GpuError::NoAdapter, never a software fallback (force_fallback_adapter: false).
• lib.rs — render_to_rgba: project a ViewState through a Renderer and rasterize in one call (GPU twin of positron_ratatui::render_to_buffer).

Why the GPU path is an example, not a #[test]

CI runners have no GPU adapter. The pure projection is the always-on unit-tested contract proof; the hardware path is proven by examples/counter_gpu.rs, which rasterizes the same Counter the tests project and writes PNGs. This honors fail-loud — no silent software fallback.

Verification

• cargo fmt --check clean, cargo clippy --workspace --all-targets -- -D warnings clean.
• Workspace tests: 31 pass (6 new in positron-wgpu; O1–O3 unchanged).
• Live on Metal: cargo run -p positron-wgpu --example counter_gpu → 3 renders three green quads, -4 four red (magnitude→count, sign→color, correct readback).

Roadmap: closes O4 → O5 is next (ContinuumHost: first real ViewState, session↔Commands/Events, reconcile RgbaFrame with continuum's RenderBackend).

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