ci: use pre-built binaries for Docker instead of QEMU cross-compilation

QEMU arm64 Rust builds took 60+ min and timed out in CI. Now the Docker
job downloads the pre-built release binaries and packages them directly.

- Dockerfile.ci: slim image for CI (downloads pre-built binaries)
- Dockerfile: full source build for local dev (unchanged build stage)
- Both use ubuntu:24.04 (GLIBC 2.39 matches CI build environment)
- Multi-arch manifest combines amd64 + arm64 images

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -46,11 +46,10 @@ RUN touch crates/*/src/*.rs \
 # ---------------------------------------------------------------------------
 # Stage 2: Minimal runtime image
 # ---------------------------------------------------------------------------
-FROM debian:bookworm-slim
+FROM ubuntu:24.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
-    libssl3 \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy both binaries