launcher: document build, run, and placeholder key in readme

README.md

@@ -1,3 +1,88 @@
 # metin-launcher
 
-Self-updating launcher for the Metin2 client (dev repo)
+Self-updating launcher for the Metin2 client.
+
+This is the single entry point the player runs. It fetches a signed manifest from
+`updates.jakubkadlec.dev`, verifies an Ed25519 signature against a compiled-in
+public key, reconciles the local client against the manifest by sha256, downloads
+anything that differs from a content-addressed blob store (with HTTP Range resume),
+applies updates atomically, hands launcher self-update off to Velopack, and then
+starts `Metin2.exe`. If the update server is unreachable the launcher falls back
+to launching whatever the player already has.
+
+Architecture and failure modes live in the client repo under
+`docs/update-manager.md` and `docs/update-manifest.md`. This project implements
+those specs in C# .NET 8; it does not own the spec.
+
+## Build
+
+```
+dotnet build Launcher.sln -c Release
+dotnet test
+```
+
+Tests are xUnit and run on Linux. A local `HttpListener` stands in for the blob
+server in `BlobDownloaderTests`.
+
+## Publish (Windows single-file)
+
+```
+dotnet publish src/Metin2Launcher/Metin2Launcher.csproj \
+    -c Release -r win-x64 --self-contained \
+    -p:PublishSingleFile=true
+```
+
+The CI hasn't been wired yet; the MVP only requires the build to succeed.
+
+## Run
+
+Drop `Metin2Launcher.exe` next to `Metin2.exe` in the client install directory
+and run it. On first launch it will:
+
+1. fetch `https://updates.jakubkadlec.dev/manifest.json` + `.sig`
+2. verify the signature against the compiled-in public key
+3. hash every file listed in the manifest, download anything that doesn't match
+   into `.updates/staging/`, then atomically move each one into place
+4. check for a Velopack launcher update against the feed at
+   `https://updates.jakubkadlec.dev/launcher` (skipped cleanly on non-installed
+   dev builds)
+5. start `Metin2.exe`
+
+A log is written to both stdout and `<client>/.updates/launcher.log`. On a plain
+`dotnet run` (no client root yet), the log falls back to
+`%LOCALAPPDATA%/Metin2Launcher/launcher.log` on Windows or
+`$XDG_DATA_HOME/Metin2Launcher/launcher.log` on Linux.
+
+## Project layout
+
+```
+src/Metin2Launcher/
+  Program.cs               orchestration + Velopack bootstrap + game launch
+  Config/LauncherConfig.cs hardcoded URLs, timeouts, public key
+  Manifest/                DTOs, HTTP loader, Ed25519 verifier
+  Transfer/                streaming sha256, Range-resume blob downloader
+  Apply/                   atomic staging -> final move
+  GameLaunch/              CreateProcess wrapper
+  Logging/                 hand-rolled file + stdout logger
+tests/Metin2Launcher.Tests/
+```
+
+## Dependencies
+
+- **NSec.Cryptography** — libsodium-backed Ed25519 (.NET 8 BCL does not have it).
+- **Velopack** — launcher self-update. Only used for the launcher binary itself;
+  game assets (~4 GB) are handled by our own patcher code.
+- **xunit / xunit.runner.visualstudio** — tests.
+
+## PLACEHOLDER public key
+
+`LauncherConfig.PublicKeyHex` currently contains a **test-only** Ed25519 public
+key generated locally during MVP scaffolding:
+
+```
+a8619c11348cb26cdcad9467c5dd44e3fcb40017e9fbcf225ac76ad824422c46
+```
+
+This MUST be replaced with the real production signing key before the first
+public release. The private half of this placeholder was used only to prove
+signature wiring works; it is not tied to any real release.