Add BUILDING.md documenting deferred build steps

docs/BUILDING.md

@@ -0,0 +1,50 @@
+# Building ubuntu-fido from source
+
+## System dependencies
+
+```bash
+sudo apt-get update
+sudo apt-get install -y \
+    build-essential pkg-config \
+    libpam0g-dev libgtk-4-dev libadwaita-1-dev libfido2-dev \
+    debhelper devscripts lintian
+```
+
+Rust toolchain: stable (1.78 minimum). Install via [rustup](https://rustup.rs).
+
+## Build steps
+
+```bash
+# Pure-Rust crates (daemon, CLI, common)
+cargo build --release
+
+# GUI (requires libgtk-4-dev + libadwaita-1-dev)
+cargo build -p ubuntu-fido-gui --release
+
+# PAM module (requires libpam0g-dev)
+make -C pam
+
+# Full deb build (requires all of the above)
+debuild -us -uc -b
+# Output: ../ubuntu-fido*.deb
+```
+
+## Tests
+
+```bash
+cargo test --workspace --exclude ubuntu-fido-gui
+```
+
+The GUI crate has no automated tests in v1; manual smoke tests are documented in [docs/plans/2026-04-26-ubuntu-fido-design.md](plans/2026-04-26-ubuntu-fido-design.md#testing-strategy).
+
+## Phase 0 status
+
+Repository scaffolding is complete (tag `v0.1.0-scaffolding`). Subsequent phases per [docs/plans/2026-04-26-ubuntu-fido-implementation.md](plans/2026-04-26-ubuntu-fido-implementation.md).
+
+Build verifications NOT yet run on this machine (require `sudo apt install libpam0g-dev libgtk-4-dev libadwaita-1-dev libfido2-dev`):
+
+- `make -C pam` — PAM module compile
+- `cargo build -p ubuntu-fido-gui --release` — GUI link
+- `debuild -us -uc -b` — full deb build
+
+Once those packages are installed, run the three commands above to complete the Phase 0 acceptance gate.