Add MAC CGO option.

Dockerfile

@@ -129,8 +129,7 @@ WORKDIR /app
 # openssl/ca-certificates: TLS cert generation + HTTPS validation.
 # wget/tar/unzip/xz-utils: required by toolchain-manager for on-demand downloads.
 # ffmpeg: server-side remote desktop recording encoder.
-# clang: fallback C compiler for darwin/CGO agent builds (no toolchain mapping in
-# toolchain-manager.ts, so build-process.ts falls back to the default `cc`).
+# clang/lld: Darwin CGO cross-compiler/linker used with a user-uploaded macOS SDK.
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
     apt-get update && apt-get install -y --no-install-recommends \
@@ -143,6 +142,7 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
         git \
         ffmpeg \
         clang \
+        lld \
     && rm -rf /var/lib/apt/lists/*
 
 # Reuse Go + garble from the builder so we don't re-download.