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/naming.md

@@ -0,0 +1,22 @@
+# Persistent naming
+
+A fillet stores the edges it rounds, and a sketch stores the face it sits on. If those were face
+and edge *indices*, an upstream edit that adds a face would move the fillet to the wrong edge.
+SolveCraft stores **names** instead (`crates/doc/src/naming.rs`):
+
+- A face of an evaluated body is named after how it was made. A face lying on a face of the
+  feature's input keeps that face's name; a face new to the feature is named by its role:
+  `F<feature>:side:<sketch curve>`, `F<feature>:start` and `:end`, `F<feature>:blend:<k>` and
+  so on.
+- Pieces of a split face get `#1`, `#2`, … in order along the longest axis.
+- An edge is named by the two faces it separates.
+- Pattern and mirror copies are named after their sources plus the instance.
+
+Names are computed lazily, the first time something asks, from the body, the feature that made
+it and the model just before that feature. Face provenance from the kernel (which input face an
+output face came from) does most of the work.
+
+When a feature evaluates, each stored reference is resolved by name. If no face has the name
+any more, the reference falls back to the geometry it recorded (a point on the face, a normal),
+and the feature shows a warning so the user can check it. A reference that matches nothing is
+an error, never a guess.