[folbricht]

Reworks the top of the README so it explains where desync fits and what an update costs in practice.

• Where it fits replaces "What it's for". Each scenario names the feature that makes it work: A/B device updates that can be restarted, one store serving a fleet through a CDN or a caching chunk-server, running an image from a FUSE mount with --cor-file, keeping every build with prune, and chunk encryption for untrusted hosting.
• What an update costs replaces "What it saves". The single pair of container root filesystems is replaced by six builds of the Debian 12 genericcloud disk image (3.2 GB raw) in one store, with a client updating to the newest build from starting points between 2 days and 1 year old. The caveat links to the existing update size estimation recipe instead of repeating its commands.
• Quick Start shows publishing two versions, a first install with a local cache, and a seeded update. It also creates the store and cache directories, which make and extract -c require.
• Key Features entries link to their documentation pages.
• Design Philosophy is renamed "Differences from casync", and its first entry now describes the actual difference in how seeds are handled.

The numbers were measured with desync make, inspect-chunks and info --seed against builds 20250909-2230 through 20260909-2596 from cloud.debian.org. Download sizes are compressed chunks.