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/kernel.md

@@ -0,0 +1,37 @@
+# The kernel boundary and truck
+
+`crates/kernel` is SolveCraft's B-rep kernel. It wraps
+[truck](https://github.com/ricosjp/truck) (Apache-2.0) for topology, NURBS geometry, booleans,
+tessellation and STEP parsing, and it is the only crate allowed to use truck. Callers get
+SolveCraft types (`Body`, face and edge indices, meshes, measures) and a
+`KernelError` instead of truck types and panics.
+
+## `guard`
+
+truck can panic on geometry it doesn't handle. Every call into it goes through
+`solvecraft_kernel::guard`, which catches the panic and turns it into
+`KernelError::Internal`, leaving the model unchanged:
+
+```rust
+pub fn guard<T>(what: &str, f: impl FnOnce() -> Result<T>) -> Result<T>
+```
+
+A failing feature shows an error on the timeline; it never takes the app down.
+
+## What is our own
+
+truck has no fillets, chamfers, shells or face edits, so these are SolveCraft code in the
+kernel: constant, chord and variable-radius fillets with corner blends, chamfers, shell and
+offset, Delete Face, Split Face, Replace Face, sewing, STEP and IGES reading and writing beyond
+truck's parser, and face provenance (which input face each output face came from, for
+[persistent naming](naming.md)).
+
+## Vendored truck crates
+
+Patched copies of `truck-shapeops`, `truck-stepio` and `truck-meshalgo` live in `vendor/`,
+wired in with `[patch.crates-io]`. The patches fix robustness bugs found by our tests and the
+STEP corpus. `vendor/` is excluded from our fmt and clippy rules, and each patch should go
+upstream when it can.
+
+Some truck assertions only run in debug builds and can fail on valid geometry, so tests and CI
+run with `--release`.