Publish Nerbus source snapshot and build documentation

BUILD.md

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+# Building Nerbus Wallet
+
+## Windows x64
+
+Install Visual Studio C++ Build Tools, Qt 6.6.2 MSVC x64, CMake, and Ninja.
+From PowerShell in this repository, download the pinned libsodium dependency:
+
+```powershell
+powershell -ExecutionPolicy Bypass -File scripts/fetch-sodium.ps1
+```
+
+From an x64 Visual Studio developer prompt:
+
+```bat
+cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=C:/Qt/6.6.2/msvc2019_64
+cmake --build build --parallel 2
+C:/Qt/6.6.2/msvc2019_64/bin/windeployqt.exe --release --compiler-runtime build/nerbus-qt.exe
+copy nerbus.svg build\nerbus.svg
+```
+
+Copy the separately built `nerbus-wallet-core.exe` beside `nerbus-qt.exe`.
+`libsodium.dll` is copied by CMake. Distribute the complete runtime folder.
+
+## Linux x86_64
+
+```sh
+sudo apt-get install build-essential cmake pkg-config qt6-base-dev libqt6svg6-dev libsodium-dev
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
+cmake --build build -j2
+cp nerbus.svg build/
+```
+
+Place a native `nerbus-wallet-core` beside `nerbus-qt`. Install Qt runtime
+dependencies or bundle them with the desktop application. The prepared Linux
+package targets Ubuntu 22.04 / glibc 2.35 and uses X11 or XWayland.
+
+## Offline signing helper
+
+Clone and build [Nerbus Core](https://git.nerbus.com/nerbus/nerbus) for the target
+platform first. `core/nerbus-wallet-core.cpp` replaces the transaction utility's
+entry point and includes its original source. The detailed procedure is in
+[BUILD.txt](BUILD.txt), section OFFLINE NRB CORE COMPONENT. Keep this helper
+separate from the standard `nerbus-tx` utility and restore the original source
+and utility after compiling it. Windows dependencies must be built with MinGW;
+the GUI uses MSVC, and they communicate over an anonymous pipe.
+
+The server bridge is in `server/`. It is not required for local address creation.