cli: snapshot draws a script's model to a PNG with no window

fieldcraft-cli snapshot script.json --out shot.png [--view] [--size WxH]
draws the engine's framed scene (engine::view::frame, shared with the CPU
render_png) with the viewport's GPU pipelines offscreen: hardware GPU, then
a software one, then the CPU rasterizer (FIELDCRAFT_GPU=auto|hardware|
software|cpu). CI installs lavapipe so the GPU path runs there.

Refs #422

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

Cargo.lock

@@ -1292,9 +1292,12 @@ name = "fieldcraft-cli"
 version = "0.6.0"
 dependencies = [
  "fieldcraft-engine",
+ "fieldcraft-gpu",
  "fieldcraft-io",
  "fieldcraft-mcp",
+ "fieldcraft-render",
  "fieldcraft-update",
+ "png",
  "serde_json",
 ]