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

Cargo.toml

@@ -28,6 +28,7 @@ fieldcraft-render = { path = "crates/render" }
 fieldcraft-ui-egui = { path = "crates/ui-egui" }
 fieldcraft-mcp = { path = "crates/mcp" }
 fieldcraft-update = { path = "crates/update" }
+fieldcraft-gpu = { path = "crates/gpu" }
 
 # foundation
 thiserror = "2"
@@ -51,6 +52,9 @@ resvg = { version = "0.48", default-features = false }
 egui = "0.36"
 eframe = { version = "0.36", default-features = false }
 egui-wgpu = "0.36"
+# the wgpu egui-wgpu uses, for the offscreen viewport (crates/gpu); backends come per target there
+wgpu = { version = "30", default-features = false, features = ["std", "wgsl"] }
+pollster = "0.4"
 # Shared Craft-family egui components (icon buttons, tabs, widget palette); the same rev as
 # VectorCraft. MIT OR Apache-2.0.
 craft-ui = { git = "https://github.com/storytold/craft-libs", rev = "5c874d51eeb3de70e735c8ce87a59a7dedd47675" }