feat: add mdbook documentation setup for github.io

- Add book/book.toml with mdbook configuration
- Add book/src/SUMMARY.md with chapter links
- Add book/src/intro.md with architecture overview

CI workflow triggers docs build on push to main/milestone/**/feature/**

Author: Claude Sonnet (coordinator)

book/book.toml

@@ -0,0 +1,44 @@
+[book]
+authors = ["l3dg3rr"]
+language = "en"
+multilingual = false
+src = "src"
+title = "l3dg3rr Ledger Documentation"
+
+[build]
+build-msrv = "1.88"
+
+[preprocessor.mdbook-embed]
+maximumEmbedDepth = 2
+
+[preprocessor.mathjax]
+
+[preprocessor.links]
+
+[output.html]
+additional-js = []
+additional-css = []
+default-theme = "light"
+edit-url-template = "https://github.com/PromptExecution/l3dg3rr/edit/main/book/src/{path}#L{line}"
+git-repository-icon = "fa-book"
+git-repository-url = "https://github.com/PromptExecution/l3dg3rr"
+google-analytics = ""
+language = "en"
+prefediting = []
+theme = "rusted"
+enable-git-edit-link = true
+
+[output.html.playground]
+editable = false
+fill-attributes = false
+run-url = ""
+
+[output.latex]
+latex-theme = ["book"]
+
+[output.mdbook-mermaid]
+skip-fold-levels = 0
+
+[output.linkcheck]
+analyze-inlinks = false
+builder = "nitpicky"

book/src/SUMMARY.md

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+# Summary
+
+- [Introduction](./intro.md)
+- [Graph Data Model](./graph.md)
+- [Force Layout](./layout.md)
+- [Isometric Projection](./iso.md)
+- [Renderer](./render.md)
+- [Slint Visualization](./slint_viz.md)
+- [Pipeline](./pipeline.md)
+- [Validation](./validation.md)
+- [Legal Verification](./legal.md)
+- [Constraints](./constraints.md)
+- [Verification](./verify.md)
+- [Workflow](./workflow.md)
+- [Visualization](./visualize.md)

book/src/intro.md

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+# Introduction
+
+`l3dg3rr` is a local-first personal financial document intelligence system focused on retroactive U.S. expat tax preparation from raw PDF statements.
+
+## Architecture
+
+The system implements a 6-layer visualization stack:
+
+| Layer | Module | Description |
+|-------|--------|-------------|
+| 0 | graph.rs | NodeData, EdgeData, pipeline node/edge vectors |
+| 1 | layout.rs | ForceLayout with Fruchterman-Reingold solver |
+| 2 | layout.rs | Isometric projection (iso_project) |
+| 3 | render.rs | GraphRenderer for screen coordinates |
+| 4 | slint_viz.rs | SlintGraphView with Arc<RwLock> |
+| 5 | host-window.rs | GraphView Slint component |
+
+## Source Documentation
+
+This book is auto-generated from Rustdoc comments in the source code.
+
+```bash
+cargo doc --workspace --no-deps
+mdbook build
+```