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/testing.md

@@ -0,0 +1,40 @@
+# Test harnesses
+
+`cargo xtask ci` runs everything a pull request must pass: fmt, clippy with `-D warnings`, the
+tests in release mode, `xtask layers`, `xtask assets` and the wasm build check.
+
+| Harness | Where | What it catches |
+|---|---|---|
+| Unit and engine tests | next to the code, `*_tests.rs` | measured results: volume, area, topology counts against derived values |
+| Hostile input | `hostile_params_never_panic`, `crates/engine/src/tests.rs` | every command × every documented parameter × junk values: errors, never panics |
+| Sketch fuzz | `crates/engine/src/cmd/sketch_fuzz_tests.rs` | random sketches through the solver and profile finder |
+| UI scenarios | `crates/ui-egui/tests/scenarios/*.json` | the real app, headless: clicks, keys, drags, assertions |
+| Layout tests | `crates/ui-egui/tests/ui_layout.rs`, `window_widths.rs` | panels and dialogs at every window width |
+| Design-file corpus | `apps/solvecraft-cli/tests/corpus` | every saved file version still opens and evaluates the same |
+| Kill mid-save | `apps/solvecraft-cli/tests/kill_mid_save.rs` | a save killed at any moment leaves a readable file |
+| Fusion oracle | `cargo xtask oracle` → [docs/oracle.md](oracle.md) | parts rebuilt from recipes, compared with Fusion's measurements |
+| STEP corpus | `cargo xtask step-corpus` → [docs/step-import.md](step-import.md) | STEP files from Fusion opened and measured |
+
+## UI scenarios
+
+A scenario is a JSON list of steps run against the real app with a headless egui context:
+
+```json
+[
+  {"start": "sketch.create"},
+  {"click": {"plane": "XY"}},
+  {"start": "sketch.rectangle.two_point"},
+  {"click": {"sketch": [0, 0]}},
+  {"click": {"sketch": [40, 30]}},
+  {"expect": {"errors": 0}}
+]
+```
+
+A new file in `crates/ui-egui/tests/scenarios/` is picked up automatically. To record a bug
+before it's fixed, add a `{"pending": "why"}` step; its test is then ignored with that reason, and
+the fix removes the step.
+
+## Release mode
+
+CI runs the tests with `--release`, because some truck assertions only exist in debug builds
+and fail on valid geometry. If a test fails only in debug, say so in the PR.