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.

README.adoc

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+= MapTool
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+
+A mapping application backend written in Ruby with Sinatra, optionally
+wrapped as a native desktop app with Tauri.
+
+The project's shape is still open — this is the initial scaffold.
+
+== Status
+
+Early scaffold. The web application serves a basic Leaflet map; the
+desktop shell boots the same server inside a native window.
+
+== Requirements
+
+* Ruby (see `.ruby-version`) and Bundler
+* Rust and the Tauri CLI, only if you want to build the desktop shell
+  (see `docs/development.adoc`)
+
+== Getting started
+
+[source,shell]
+----
+bundle install
+bundle exec puma -C config/puma.rb
+----
+
+Then open http://127.0.0.1:4567 in a browser.
+
+== Documentation
+
+* link:docs/architecture.adoc[Architecture]
+* link:docs/development.adoc[Development guide]