house: add Linux AMD64 build guide

- Add BUILD_LINUX.md with instructions for building .deb and AppImage
  packages on Linux AMD64 from the source distribution
- Document prerequisites, build steps, and usage for each package type
- Include source distribution (sdist) and pure Python wheel in dist/

BUILD_LINUX.md

@@ -0,0 +1,133 @@
+# UmbraEdit - Linux AMD64 Build Guide
+
+UmbraEdit is built on macOS (darwin/aarch64), so native Linux packages (.deb, .rpm, AppImage)
+cannot be built directly from this machine. This document describes how to build
+Linux AMD64 install packages from the source artifacts provided here.
+
+## Build Artifacts
+
+The following files are in `dist/`:
+
+| File | Description | Purpose |
+|------|-------------|---------|
+| `umbraedit-2026.11.1.tar.gz` | Source distribution (sdist) | Source tarball for building on Linux |
+| `umbraedit-2026.11.1-py3-none-any.whl` | Pure Python wheel | Installable via pip on any platform |
+
+## Prerequisites on Linux AMD64
+
+Before building, install the required system packages:
+
+```bash
+# Debian/Ubuntu
+sudo apt-get update
+sudo apt-get install python3 python3-pip python3-pyqt6 python3-pyqt6.qtsvg \
+    python3-enchant qt6-image-formats-plugins qt6-svg-plugins \
+    dh-python debhelper python3-build python3-setuptools
+
+# Fedora/RHEL
+sudo dnf install python3 python3-pip python3-pyqt6 python3-enchant \
+    qt6-qtimageformats qt6-qtsvg python3-build python3-setuptools
+
+# Arch Linux
+sudo pacman -S python python-pip python-pyqt6 python-enchant qt6-imageformats qt6-svg \
+    python-build python-setuptools
+```
+
+## Option 1: Install via Wheel (Simplest)
+
+The pure Python wheel can be installed directly on Linux AMD64 with pip:
+
+```bash
+# Copy the wheel to your Linux AMD64 machine
+scp umbraedit-2026.11.1-py3-none-any.whl user@linux-machine:~/
+
+# On the Linux machine
+pip install umbraedit-2026.11.1-py3-none-any.whl
+
+# Run UmbraEdit
+umbraedit
+```
+
+This installs the application as a Python package with the `umbraedit` console script
+entry point. No compilation is required.
+
+## Option 2: Build from Source Distribution
+
+The source tarball contains all source files needed to build native Linux packages:
+
+```bash
+# Copy the source tarball to your Linux AMD64 machine
+scp umbraedit-2026.11.1.tar.gz user@linux-machine:~/
+
+# On the Linux machine
+tar xzf umbraedit-2026.11.1.tar.gz
+cd umbraedit-2026.11.1
+
+# Build and install locally
+pip install .
+```
+
+## Option 3: Build Debian (.deb) Package
+
+On a Debian/Ubuntu-based Linux AMD64 system:
+
+```bash
+# Install build dependencies
+sudo apt-get install dh-python pybuild-plugin-pyproject python3-build \
+    python3-setuptools python3-all debhelper \
+    python3-pytest python3-pytestqt python3-pytest-timeout
+
+# Extract and build
+tar xzf umbraedit-2026.11.1.tar.gz
+cd umbraedit-2026.11.1
+
+# Build .deb package
+python3 utils/build_debian.py --distro noble  # or trixie, resolute, stonking
+
+# The .deb will be in dist_deb/
+sudo dpkg -i dist_deb/umbraedit_*.deb
+```
+
+Available distro targets: `bookworm`, `trixie`, `noble`, `resolute`, `stonking`, `wilma`
+
+## Option 4: Build AppImage
+
+On any Linux AMD64 system:
+
+```bash
+# Install AppImage build tools
+pip install python-appimage
+sudo apt-get install appimagetool  # or download from GitHub
+
+# Extract and build
+tar xzf umbraedit-2026.11.1.tar.gz
+cd umbraedit-2026.11.1
+
+# Build AppImage
+python3 utils/build_appimage.py --linux ubuntu --arch amd64 --python 3.12
+
+# The AppImage will be in dist_appimage/
+chmod +x dist_appimage/umbraedit-2026.11-amd64.AppImage
+./dist_appimage/umbraedit-2026.11-amd64.AppImage
+```
+
+## Project Storage
+
+UmbraEdit projects are stored in plain folders with an `nwProject.nwx` file.
+The sample project included in the distribution can be used for testing:
+
+```bash
+umbraedit  # Opens the welcome dialog
+# Select "Sample Project" from the list, or browse to your .nwx file
+```
+
+## Version
+
+Current development version: **2026.11.0003**
+
+## License
+
+GPL-3.0-or-later AND Apache-2.0 AND ISC AND MIT
+
+UmbraEdit is based on novelWrite 2026.02 by Veronica Berglyd Olsen
+(novelWriter.io), licenced under the same GPL v3.0 license.