gpu: the viewport's pipelines move to a crate of their own (fieldcraft-gpu)

ui-egui keeps only the egui_wgpu paint callback; the shaders, batching and
pipelines are fieldcraft-gpu's view module, so an offscreen picture can draw
with the window's pipelines.

Refs #422

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/gpu/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "fieldcraft-gpu"
+description = "FieldCraft's GPU viewport pipelines (lit faces, wide lines) and offscreen captures: the window's picture, drawn without a window"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+fieldcraft-render = { workspace = true }
+wgpu = { workspace = true }
+log = { workspace = true }
+thiserror = { workspace = true }
+
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+pollster = { workspace = true }
+# Native backends, so the CLI and the tests open a device without a window.
+wgpu = { workspace = true, features = ["metal", "vulkan", "dx12", "gles"] }