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

.github/workflows/ci.yml

@@ -132,6 +132,8 @@ jobs:
           # Shared keys: release.yml's verify jobs restore these caches (they save only on main).
           shared-key: ci
           save-if: ${{ github.ref == 'refs/heads/develop' }}
+      - name: Software GPU (lavapipe), so headless snapshots run the GPU pipeline
+        run: sudo apt-get update -q && sudo apt-get install -y -q --no-install-recommends mesa-vulkan-drivers
       - name: cargo xtask ci (the affected crates, or everything)
         env:
           BASE: ${{ needs.plan.outputs.base }}