feat(sim): add MediaMTX binary downloader

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

scripts/download_mediamtx.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# Download the MediaMTX static binary used by scripts/sim_cameras.sh.
+# One-time setup. Re-run to upgrade.
+set -euo pipefail
+
+VERSION="${MEDIAMTX_VERSION:-1.9.3}"
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+DEST_DIR="${REPO_ROOT}/.sim"
+DEST="${DEST_DIR}/mediamtx"
+
+case "$(uname -s)_$(uname -m)" in
+  Linux_x86_64)  ASSET="mediamtx_v${VERSION}_linux_amd64.tar.gz" ;;
+  Linux_aarch64) ASSET="mediamtx_v${VERSION}_linux_arm64v8.tar.gz" ;;
+  Darwin_x86_64) ASSET="mediamtx_v${VERSION}_darwin_amd64.tar.gz" ;;
+  Darwin_arm64)  ASSET="mediamtx_v${VERSION}_darwin_arm64.tar.gz" ;;
+  *)
+    echo "Unsupported platform: $(uname -s) $(uname -m)" >&2
+    exit 1
+    ;;
+esac
+
+URL="https://github.com/bluenviron/mediamtx/releases/download/v${VERSION}/${ASSET}"
+
+mkdir -p "${DEST_DIR}"
+TMP="$(mktemp -d)"
+trap 'rm -rf "${TMP}"' EXIT
+
+echo "Downloading ${ASSET}..."
+curl -fsSL "${URL}" -o "${TMP}/mediamtx.tar.gz"
+
+echo "Extracting..."
+tar -xzf "${TMP}/mediamtx.tar.gz" -C "${TMP}"
+
+mv "${TMP}/mediamtx" "${DEST}"
+chmod +x "${DEST}"
+
+echo "Installed: ${DEST}"
+"${DEST}" --version || true