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 <[email protected]>

xtask/src/main.rs

@@ -4,6 +4,7 @@
 //! `std::process::Command`.
 
 mod assets;
+mod book;
 mod layers;
 mod parity;
 
@@ -24,6 +25,9 @@ commands:
   step-corpus [DIR]
                   import every DIR/*/part.step (default plan/fusion/oracle) and compare
                   with measure.json; writes docs/step-import.md
+  book [--no-build]
+                  generate docs/book's command reference and screenshots, then mdbook build
+                  into target/book
   wasm            cargo check every library crate and the web app for wasm32-unknown-unknown
   ci              fmt --check, clippy -D warnings, test, assets, layers, wasm (stops at first failure)
 ";
@@ -37,6 +41,7 @@ fn main() -> ExitCode {
         Some("parity") => parity::run(&root(), rest.contains(&"--refresh")),
         Some("oracle") => cmd_oracle(),
         Some("step-corpus") => cmd_step_corpus(rest.first().copied()),
+        Some("book") => book::run(&root(), !rest.contains(&"--no-build")),
         Some("ci") => cmd_ci(),
         Some("wasm") => cmd_wasm(),
         Some("-h" | "--help" | "help") | None => {