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

docs/architecture.md

@@ -32,6 +32,7 @@ crates in strictly lower layers.
 | L4 | `fieldcraft-doc` | The document: geometry, mesh, model, results, history. | planned |
 | L5 | `fieldcraft-io` | `.fieldcraft` files (docs/formats/fieldcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery; read and write roots that confine an automation client's file access (`roots`). Bytes in, bytes out; the file system only in `fs`, `recovery` and `roots` (in the browser, files kept in memory: `memory`). Result files later. | started |
 | L6 | `fieldcraft-engine` | Commands (`CommandSpec`), `Session::execute` with rollback and the panic guard, scripts; every file a command names checked against the session's roots (`Session::file_access`); headless pictures of the session (`view`: model, geometry, deformed shape and contours on the CPU rasterizer). | started |
+| L7 | `fieldcraft-gpu` | The viewport's GPU pipelines (lit faces, wide lines, MSAA, depth) without a window toolkit, the adapter policy (`FIELDCRAFT_GPU`: hardware GPU, software GPU, CPU rasterizer) and offscreen pictures (`fieldcraft-cli snapshot`). The window draws through the same pipelines. | started |
 | L7 | `fieldcraft-ui-egui`, `fieldcraft-mcp` | Desktop UI in the Craft family's layout (title bar with quick actions, workspace ribbon from the catalogue, browser, viewport with navigation bar and orientation gizmo, context panel, command search and command line, journal timeline; native file pickers as host services, background opening and solving; results display with legend and probe; offscreen capture of the whole window) and control channel requests (docs/control-protocol.md); MCP server over stdio (docs/mcp.md): tools over engine commands, a headless session or the running app through its control channel, read and write roots. | ui, mcp started |
 | testkit | `fieldcraft-testkit` | Test tooling, a dev-dependency only: the meshing test corpus (`tests/corpus/*/recipe.json`, analytic shapes) and its runner (`cargo xtask corpus`). | started |
 | apps | `fieldcraft`, `fieldcraft-cli`, `fieldcraft-web` | Desktop, command line, browser (docs/web.md). | started |