Documentation site and CI workflows

An mdBook under docs/book: a user guide (getting started, UI tour, sketching, features,
parameters, components and joints, sheet metal, plastic, files, shortcuts, MCP) and a developer
guide (CONTRIBUTING, architecture, command engine, kernel boundary, persistent naming, test
harnesses, and the existing docs/*.md pages). `cargo xtask book` generates the command reference
from `solvecraft-cli commands` and renders the screenshots with `solvecraft-cli snapshot`, then
runs mdbook. .github/workflows/pages.yml publishes the book at /solvecraft/ and the web app at
/solvecraft/app/ (skipped while the repository is private). .github/workflows/ci.yml runs
`cargo xtask ci` on pull requests. README and Help → Documentation link to the book.

Fixes #28
Fixes #8

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/dev/architecture.md

@@ -0,0 +1,22 @@
+# Architecture
+
+SolveCraft is a Cargo workspace in layers. A crate only depends on crates in lower layers, and
+`cargo xtask layers` fails the build otherwise.
+
+{{#include ../../../../README.md:architecture}}
+
+The web app, `apps/solvecraft-web`, is the desktop front end compiled to wasm with
+[trunk](https://trunkrs.dev). `xtask` holds the repository's own tooling: `cargo xtask ci`,
+`layers`, `assets`, `oracle`, `parity`, `step-corpus` and `book`.
+
+A few rules follow from the layering:
+
+- **Only `crates/kernel` touches truck.** Everything above it sees SolveCraft's own types:
+  bodies, faces, edges, meshes and measures. See [The kernel boundary](kernel.md).
+- **The document knows nothing about commands or the UI.** `crates/doc` holds the parameters,
+  the feature timeline and the component tree, and evaluates them into a model.
+- **Everything the user can do is a command** in `crates/engine`. See
+  [The command engine](commands.md).
+- **The front end is swappable.** Nothing below `ui-egui` depends on egui. The CLI and the MCP
+  server run the same engine without a window, and `crates/render` has a CPU rasterizer so
+  headless screenshots work anywhere.