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/src/lib.rs

@@ -0,0 +1,31 @@
+//! FieldCraft on the GPU without a window toolkit: the viewport's pipelines ([`view`]), the
+//! choice of adapter ([`select`]) and offscreen pictures drawn with them ([`capture`]). The
+//! desktop viewport draws through [`view`] in its paint callback; `fieldcraft-cli snapshot` draws
+//! through [`capture`], so a headless picture is the one the window draws.
+#![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
+#![forbid(unsafe_code)]
+
+#[cfg(not(target_arch = "wasm32"))]
+pub mod capture;
+#[cfg(not(target_arch = "wasm32"))]
+pub mod select;
+pub mod view;
+
+pub use wgpu;
+
+/// What can go wrong on the GPU. Never a panic: device errors are caught in error scopes.
+#[derive(Debug, Clone, PartialEq, thiserror::Error)]
+pub enum GpuError {
+    #[error("{0}")]
+    NoAdapter(String),
+    #[error("FIELDCRAFT_GPU must be auto, hardware, software or cpu, not `{0}`")]
+    Policy(String),
+    #[error("the GPU device could not be opened: {0}")]
+    Device(String),
+    #[error("{0}")]
+    Size(String),
+    #[error("the GPU picture failed: {0}")]
+    Failed(String),
+}
+
+pub type Result<T> = std::result::Result<T, GpuError>;