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/guide/features.md

@@ -0,0 +1,24 @@
+# Solid features
+
+Features live on the **timeline** and rebuild incrementally from the first one that changed.
+Each one can be edited (double-click), suppressed, renamed, reordered (if nothing it depends on
+moves after it), rolled back to, and deleted. Deleting asks first when other features depend on
+it.
+
+| Panel | Features |
+|---|---|
+| Create | extrude (one side, two sides, symmetric, taper, start offset, to a distance or through all; new body, join, cut, intersect), revolve, sweep, loft, box, cylinder, sphere, torus, coil, pipe, rib, web, emboss, hole (simple, counterbore, countersink, tapped; on faces or at sketch points), thread, rectangular/circular/path patterns, mirror |
+| Modify | press pull, fillet, chamfer, shell, draft, scale, combine, split body, offset face, replace face, move/copy, align, remove |
+| Construct | offset plane, plane at angle |
+| Inspect | measure, section analysis, interference, curvature comb and map, zebra, draft, minimum radius, isocurves, centre of mass |
+
+Fillets and chamfers handle straight edges between planar faces, whole smooth loops, runs of
+neighbouring edges with mitred or spherical corners, and closed chains of curved edges (a rolling
+ball). See the roadmap's known limitations for the rest.
+
+Feature references to faces and edges are stored by name, so an upstream edit that moves geometry
+keeps them on the right face (see [Persistent naming](../dev/naming.md)). When a reference can't
+be found, the feature warns instead of silently picking another face.
+
+Press **Delete** on anything selected (bodies, faces, sketches, features, planes, components,
+canvases). It runs as one undoable step and lists the features that would go with it or fail.