Add project documentation and repository structure

docs/ARCHITECTURE.md

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+# Hudra Architecture
+
+## Overview
+
+Hudra is a Manifest V3 browser extension that presents the existing DOM of an
+HTML lesson.
+
+It does not convert the lesson into a proprietary format.
+
+## Slide detection
+
+The current presentation model uses HTML headings:
+
+- H1 identifies the lesson title
+- H2 identifies the beginning of a new slide
+- content following an H2 belongs to that slide until the next H2
+
+## Main components
+
+### background.js
+
+Handles extension-level behaviour and presentation launch logic.
+
+### controls.html
+
+Provides the presenter dashboard interface.
+
+### controls.css
+
+Styles the presenter controls.
+
+### controls.js
+
+Manages presenter navigation, controls and audience communication.
+
+### presenter.js
+
+Creates and manages the audience presentation experience.
+
+### manifest.json
+
+Defines the extension metadata, permissions and version.
+
+## Design constraints
+
+Hudra should avoid:
+
+- cloning complex media unnecessarily
+- fragile LMS-specific DOM assumptions
+- large content-detection systems
+- destructive modification of the original lesson
+
+New media features should be implemented and tested independently.