diff --git a/.cargo/config.toml b/.cargo/config.toml
index 8c295de..c6446a7 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -1,3 +1,20 @@
+# Build configuration for gopher64-netplay.
+#
+# Portability note (issue #16): the DEFAULT build must run on the review
+# box (Intel i7-2600K, Sandy Bridge 2011 = x86-64-v2). Two upstream
+# defaults broke it:
+#
+#   1. `-C target-cpu=x86-64-v3` (requires AVX2/BMI2, Haswell 2013+)
+#      miscompiles BUILD SCRIPTS on pre-AVX2 hosts: garbage allocation
+#      sizes and SIGABRT (observed: num-traits' build script aborting on
+#      a 648518346341351440-byte allocation). It is now opt-in via
+#      RUSTFLAGS for capable hardware; the default build is portable.
+#   2. `AR=llvm-ar` forced an LLVM archiver that is not in the provisioned
+#      root; dropped — the platform `ar` archives clang objects fine.
+#      `CC=clang`/`CXX=clang++` are retained (build.rs needs clang for
+#      `-flto=thin`) and resolved through PATH; see scripts/gate.sh for the
+#      review box's clang-18 wiring.
+
 [target.'cfg(all(target_arch = "aarch64", target_os = "linux"))']
 rustflags = ["-C", "link-arg=-fuse-ld=lld"]
 
@@ -7,21 +24,31 @@ linker = "rust-lld"
 [target.'cfg(target_os = "windows")']
 rustflags = ["-C", "link-arg=/STACK:8388608"]
 
-[target.'cfg(target_arch = "x86_64")']
-rustflags = ["-C", "target-cpu=x86-64-v3"]
+# No x86_64 rustflags by default: the portable baseline only requires
+# x86-64-v2 (SSE4.2). On AVX2-capable hardware, opt into the optimised
+# target with:
+#   RUSTFLAGS="-C target-cpu=x86-64-v3" cargo build --release
+# (RUSTFLAGS REPLACES the config's rustflags, so carry any other flags.)
 
 [target.'cfg(all(target_arch = "aarch64", target_os = "macos"))']
 rustflags = [
     "-C", "target-cpu=apple-m1",
-    "-C", "link-arg=-headerpad_max_install_names"
+    "-C", "link-arg=-headerpad_max_install_names",
 ]
 
 [target.'cfg(all(target_arch = "aarch64", not(target_os = "macos")))']
 rustflags = ["-C", "target-cpu=cortex-a76"]
 
 [env]
+# The C/C++ compiler is pinned to clang because build.rs compiles the
+# parallel-rdp / retroachievements C sources with clang-only flags
+# (`-flto=thin`, cf. build.rs lines ~150-153). The `cc`/`cc-rs` crate
+# resolves clang via PATH, so hosts without it fail loudly at the C build
+# (issue #16: the review box runs the clang-18 shipped in the user-local
+# LLVM root — see scripts/gate.sh). AR is intentionally NOT pinned: the
+# platform `ar` (GNU binutils) archives clang objects fine, and llvm-ar is
+# not present in the provisioned root.
+SDKROOT = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk"
 CC = "clang"
 CXX = "clang++"
-AR = "llvm-ar"
-SDKROOT = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk"
 CMAKE_POLICY_VERSION_MINIMUM = "3.5" # for freetype
diff --git a/docs/build-gate.md b/docs/build-gate.md
new file mode 100644
index 0000000..670d507
--- /dev/null
+++ b/docs/build-gate.md
@@ -0,0 +1,82 @@
+# Build gate (review box)
+
+The project's review gate is a single documented command — see
+[scripts/gate.sh](../scripts/gate.sh):
+
+```sh
+./scripts/gate.sh        # == git submodule update --init --recursive
+                         #    + cargo check --all-targets
+```
+
+It must exit 0 on a clean `origin/main` checkout before any PR is gated
+(issue #16: all open PRs were unverifiable because `main` itself did not
+build).
+
+## Why the defaults were changed (`.cargo/config.toml`)
+
+The fork's original config had two problems on older / non-LLVM hosts:
+
+1. `-C target-cpu=x86-64-v3` for `x86_64` — requires AVX2/BMI2 (Intel
+   Haswell, 2013+). On a pre-AVX2 host (the review box is Sandy Bridge,
+   `x86-64-v2`) cargo applies the target rustflags to **build scripts**,
+   which then miscompile: `num-traits`'s build script aborted with
+   `memory allocation of 648518346341351440 bytes failed` (0x0900000000000010
+   — garbage from executing AVX2 instructions, not real OOM).
+   The default build is now portable; the optimised target is opt-in:
+   `RUSTFLAGS="-C target-cpu=x86-64-v3" cargo build --release`
+   (RUSTFLAGS replaces config rustflags — carry other flags across).
+2. `[env] AR=llvm-ar` forced an LLVM archiver that is not installed in the
+   provisioned root — dropped; the platform `ar` (GNU binutils) archives
+   clang objects fine. `CC=clang`/`CXX=clang++` are **retained** because
+   `build.rs` compiles the parallel-rdp / retroachievements C sources with
+   clang-only flags (`-flto=thin`); `cc-rs` resolves `clang` via PATH, so a
+   host simply needs clang on PATH (the review box gets it from the
+   user-local LLVM root via `gate.sh`).
+   `CMAKE_POLICY_VERSION_MINIMUM=3.5` is retained (cmake >= 4 needs it for
+   freetype); `SDKROOT` is retained for macOS SDK discovery.
+
+## Host prerequisites (Linux)
+
+- `cargo`/`rustc` (toolchain pinned in `rust-toolchain.toml`)
+- `clang`/`clang++` on PATH (build.rs, see above)
+- `cmake` (SDL3, freetype), `ar`, a real `pkg-config`
+- `fontconfig` dev (the GUI text stack pulls `yeslogic-fontconfig-sys`
+  through `slint` → `parley` → `fontique`) and X11 dev (SDL3's video
+  backend; the fork's own `src/ui.rs` uses `sdl3_sys::video` directly)
+- `libclang` (`LIBCLANG_PATH`) for the `bindgen` build-dependency's
+  `clang-sys` probe
+
+The review box has **no system packages for any of these and no sudo**, so
+user-local roots extracted from Ubuntu `.debs` are provisioned:
+
+- `~/local-tools/root/usr/lib/llvm-18/bin` — clang/clang++/clang-18
+  (plus `cmake`, `ninja`, `pkgconf`, `wayland-scanner` in
+  `~/local-tools/root/usr/bin`), `libclang-18.so` in
+  `~/local-tools/root/usr/lib/llvm-18/lib`
+- `~/.local/bin-fc/pkg-config` — wrapper that execs the real `pkgconf`
+  (the pypi `pkg-config`/`pkgconf-pypi` shims on `~/.local/bin` wrongly
+  report exit 0 for missing packages, which makes cmake enable optional
+  SDL backends)
+- `~/.local/fcroot/usr/lib/x86_64-linux-gnu/pkgconfig` — `fontconfig.pc`,
+  `freetype2.pc`, `zlib.pc`, `expat.pc`, `libpng16.pc`, X11 pc files, with
+  matching headers and shared libs in the same root
+
+`scripts/gate.sh` wires all of these onto PATH/env when present; on hosts
+with system toolchains the provisioning blocks are no-ops.
+
+## Troubleshooting
+
+- `memory allocation of 648518346341351440 bytes failed` (SIGABRT in a
+  dependency build script) — an x86-64-v3 (or other AVX2-only) flag is
+  being applied on a pre-AVX2 CPU. Remove the flag / use the portable
+  default.
+- `cc1: error: unrecognized argument to '-flto=' option: 'thin'` —
+  `build.rs` requires clang; gcc rejects `-flto=thin`. Put clang on PATH
+  (or run via `scripts/gate.sh` on the review box).
+- `failed to find tool "clang"` inside `ring`'s build — clang is missing
+  from PATH.
+- `Package 'fontconfig' not found` — PKG_CONFIG_PATH is not pointing at a
+  fontconfig dev root (or the pypi pkg-config shim is intercepting).
+- SDL cmake: `SDL could not find X11 or Wayland development libraries` —
+  CMake's FindX11 needs `CMAKE_INCLUDE_PATH`/`CMAKE_LIBRARY_PATH` pointed
+  at the X11 dev root (pkg-config alone is not consulted for X11).
diff --git a/scripts/gate.sh b/scripts/gate.sh
new file mode 100755
index 0000000..35b6ade
--- /dev/null
+++ b/scripts/gate.sh
@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# gopher64-netplay review gate (issue #16)
+#
+# Documented gate command (from a clean origin/main checkout):
+#   ./scripts/gate.sh
+#
+# What it does:
+#   1. `git submodule update --init --recursive` (parallel-rdp,
+#      retroachievements/rcheevos, src/compat/sse2neon) — required by
+#      build.rs.
+#   2. Wires the review box's user-local provisioning onto PATH/env (only
+#      when present; a no-op on hosts with system toolchains):
+#        - clang/clang++ from the local LLVM-18 root  (build.rs requires
+#          clang for `-flto=thin`)
+#        - a real pkgconf + fontconfig/X11 dev pc files (the box has no
+#          system pkg-config/fontconfig and no sudo)
+#   3. Runs `cargo check --all-targets`.
+#
+# On AVX2-capable hardware, opt into the optimised CPU target:
+#   RUSTFLAGS="-C target-cpu=x86-64-v3" ./scripts/gate.sh
+# (RUSTFLAGS REPLACES the config's rustflags for the whole invocation, so
+# carry any other flags you need.)
+set -euo pipefail
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$REPO_ROOT"
+
+git submodule update --init --recursive
+
+# --- Review-box provisioning ----------------------------------------------
+LLVM_BIN="$HOME/local-tools/root/usr/lib/llvm-18/bin"
+LLVM_BIN2="$HOME/local-tools/root/usr/bin"
+FCPC="$HOME/.local/bin-fc/pkg-config"
+FCROOT="$HOME/.local/fcroot/usr"
+PC_DIR="$FCROOT/lib/x86_64-linux-gnu/pkgconfig"
+
+if [ -d "$LLVM_BIN" ]; then
+    export PATH="$LLVM_BIN:$PATH"          # clang / clang++ (build.rs needs clang)
+fi
+if [ -d "$LLVM_BIN2" ]; then
+    export PATH="$LLVM_BIN2:$PATH"         # cmake, ninja, pkgconf, wayland-scanner
+fi
+if [ -x "$FCPC" ] && [ -d "$PC_DIR" ]; then
+    export PATH="$(dirname "$FCPC"):$PATH" # real pkg-config (not the pypi shim)
+    export PKG_CONFIG_PATH="$PC_DIR"
+    # SDL's cmake resolves X11 via CMake's classic FindX11 module (path
+    # scan), not through pkg-config — point those scans at the root too.
+    export CMAKE_INCLUDE_PATH="$FCROOT/include${CMAKE_INCLUDE_PATH:+:$CMAKE_INCLUDE_PATH}"
+    export CMAKE_LIBRARY_PATH="$FCROOT/lib/x86_64-linux-gnu${CMAKE_LIBRARY_PATH:+:$CMAKE_LIBRARY_PATH}"
+    export LD_LIBRARY_PATH="$FCROOT/lib/x86_64-linux-gnu${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+fi
+if [ -d "$HOME/local-tools/root/usr/lib/llvm-18/lib" ]; then
+    export LIBCLANG_PATH="$HOME/local-tools/root/usr/lib/llvm-18/lib" # bindgen/clang-sys
+    export LD_LIBRARY_PATH="$HOME/local-tools/root/usr/lib/llvm-18/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+fi
+
+exec cargo check --all-targets