Desktop app: egui/wgpu viewport, model tree, commands and control channel (#9)

* render: display data from the FE model, orbit camera and CPU rasterizer

modelcraft-render (L1, no GPU): DisplayMesh (the unshared faces of solid
elements, shells, element edges, line elements, nodes), the orbit camera
and a CPU rasterizer with PNG output for headless snapshots, adapted from
SolidCraft's render crate. Workspace: egui/eframe/egui-wgpu 0.36 and
SolidCraft's patched egui-wgpu from the pinned rev.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: sample model script (a hex8 block and a tet4 cube)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* Desktop app: egui/wgpu viewport, model tree, commands, status bar, control channel

modelcraft-ui-egui (L5): ModelApp over the engine Session; GPU viewport
(lit faces, element edges, nodes) with a CPU fallback; orbit/pan/zoom,
standard views and fit; model tree; commands panel generated from the
registry with a JSON parameter form; status bar; control request
handling (engine.*, ui.*, ui.screenshot, ui.render). apps/modelcraft:
eframe window, --sample, --control PORT loopback JSON-lines server.
Adapted from SolidCraft's ui-egui and app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* THIRD-PARTY-LICENSES: the desktop app's dependencies

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: control protocol, desktop app in architecture, roadmap and attribution

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* render: simplify the display mesh (sorted face keys over maps, no unused fields)

Interior faces are found by sorting fixed-size face keys instead of a BTreeMap of Vecs, edges
are deduplicated by sort, and each element's corners are looked up once. Drops the unused
`element` fields on DisplayTri/DisplayLine, the unused Mat4::IDENTITY and corner_count, and
hoists the line colour out of the raster loop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* viewport: split GPU geometry into batches within the buffer limit; refuse oversized CPU renders

A mesh with a few million boundary triangles made one vertex buffer larger than the device's
max_buffer_size (256 MiB under the limits eframe requests), and wgpu's default error handler
panicked. Geometry is now uploaded in batches of whole primitives, each at most 128 MiB and the
device limit, and all are drawn. A device error handler logs errors and shows them in the status
bar instead of panicking.

The CPU renderer refuses sizes over 8192 per side or 16 MP, and scenes over 20 million
primitives, with an error, and allocates its buffers fallibly. ui.render reports a size out of
range instead of clamping it; the CPU viewport fits a large window into the limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

ROADMAP.md

@@ -12,5 +12,5 @@ LS-PrePost, with hexahedral meshing as the centre of gravity, plus its own linea
 | M4 Model definition | Materials, sections, sets and surfaces, boundary conditions, loads, steps, output requests | planned |
 | M5 Solver decks | Abaqus input, Nastran bulk data, LS-DYNA keyword: writers first, then readers | planned |
 | M6 Linear solver | Element library, sparse assembly, linear static and modal analysis, verified against analytic solutions | planned |
-| M7 Desktop and agents | egui desktop app, control channel, MCP server, browser build | planned |
+| M7 Desktop and agents | egui desktop app, control channel, MCP server, browser build | started: desktop app (viewport, model tree, commands, status bar) and control channel |
 | M8 Post-processing | Results display: deformed shape, contours, probes | planned |