Initial scaffold: Sinatra map app with Tauri desktop shell

Sinatra backend serving a basic Leaflet map, wrapped in a Tauri
desktop shell that starts the same server in a native window.

docs/architecture.adoc

@@ -0,0 +1,81 @@
+= Architecture
+:toc:
+:toclevels: 2
+
+== Overview
+
+MapTool is a Sinatra web application. The desktop build does not
+reimplement anything natively — it wraps the same Sinatra app in a
+https://tauri.app[Tauri] window, which is the standard way to ship a
+non-JavaScript backend as a desktop app with Tauri: the Rust shell owns
+the native window and starts the web server as a local process; the
+window then simply loads `http://127.0.0.1:4567` as if it were a
+regular page.
+
+[source]
+----
++----------------------------+
+| Tauri window (WebView)     |
+|  loads http://127.0.0.1:PORT
++--------------+-------------+
+               |
+               v
++----------------------------+
+| Sinatra app (Puma)         |
+|  app.rb / config.ru        |
++----------------------------+
+----
+
+== Directory layout
+
+[cols="1,3"]
+|===
+| Path | Purpose
+
+| `app.rb`, `config.ru`
+| Sinatra application and Rack entry point.
+
+| `config/puma.rb`
+| Puma server configuration (binds to `127.0.0.1` only — the app is
+  local-only, not exposed on the network).
+
+| `public/`, `views/`
+| Frontend assets and Sinatra templates. The map view uses Leaflet.
+
+| `src-tauri/`
+| The Tauri (Rust) desktop shell.
+
+| `docs/`
+| Project documentation, in AsciiDoc.
+|===
+
+== Desktop shell (Tauri)
+
+In *development*, `src-tauri/tauri.conf.json` sets `beforeDevCommand`
+to start Puma, and `devUrl` points the window at it. Running the dev
+build (see `docs/development.adoc`) starts both together.
+
+In a *packaged build* there is no `beforeDevCommand` hook, so
+`src-tauri/src/lib.rs` starts the server itself (`bundle exec puma`)
+next to the bundled resources when the app launches, and stops it when
+the window closes.
+
+=== Known limitation: production packaging
+
+The packaged-build server spawn currently assumes a Ruby + Bundler
+installation already present on the machine running the app. That is
+fine for internal or developer builds, but not for a distributable,
+self-contained desktop app.
+
+Shipping a real self-contained build would mean embedding a Ruby
+runtime as a Tauri https://tauri.app/develop/sidecar/[sidecar] binary
+(for example, produced with https://github.com/tebako/tebako[tebako]),
+instead of relying on the system Ruby. That decision — and the
+resulting sidecar build step — is not implemented yet and should be
+revisited before any real distribution build.
+
+== Networking
+
+The Sinatra server only binds to `127.0.0.1`. It is not meant to be
+reachable from other machines; the Tauri window talks to it over
+loopback only.