docs: document libclang-dev as a mandatory Linux build prerequisite

Linux source builds pull in rustables, whose build script runs bindgen
to generate nftables bindings for the LAN gateway. bindgen needs
libclang.so on the build host, so a clean source build fails with
'Unable to find libclang' unless libclang-dev (or llvm) is installed.

The prerequisite text in README.md and CONTRIBUTING.md previously
listed only the optional BLE dependencies, and packaging/README.md had
no source-build prerequisite list at all. Document libclang-dev as a
mandatory Linux build dependency, distinct from the optional BLE deps,
and note that it is build-time only so pre-built .deb installs are
unaffected.

packaging/README.md

@@ -15,6 +15,28 @@ make zip        # Windows .zip package
 make all        # deb + tarball (default)
 ```
 
+## Build Prerequisites
+
+These targets build FIPS from source, so the host needs a build
+environment in addition to a Rust toolchain (the version pinned in
+`rust-toolchain.toml` is auto-installed by rustup).
+
+On Linux, `libclang` is **required**: the LAN gateway's nftables
+bindings are generated by `bindgen` at build time, which needs
+`libclang.so` on the build host. Without it the build fails inside the
+`rustables` crate with an "Unable to find libclang" error.
+
+```sh
+sudo apt install libclang-dev    # Debian / Ubuntu
+```
+
+This is a build-time prerequisite only — it is not a runtime
+dependency, so hosts installing a pre-built `.deb` do not need it.
+
+BLE support is optional and, when building with it, additionally needs
+`bluez`, `libdbus-1-dev`, and `pkg-config`; the build picks up BLE if
+those are present and skips it cleanly if not.
+
 ## Directory Structure
 
 ```text