ci(release): build Linux artifacts as static musl binaries (#447)

Build Linux release artifacts as statically linked musl binaries for x86_64 and aarch64, update packaging and updater handling, and document the reduced runtime requirements.

Keep release version preparation locked without refreshing dependencies or committing Cargo.lock.

Closes #440

dist/release/build_musl.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env sh
+# Builds a static musl termscp release binary for the given target triple
+# inside a pinned Alpine container.
+#
+# Usage: dist/release/build_musl.sh <target-triple>
+set -eu
+
+IMAGE="rust:1.98-alpine3.22"
+
+TARGET="${1:-}"
+if [ -z "$TARGET" ]; then
+  echo "usage: $0 <target-triple>" >&2
+  exit 2
+fi
+
+case "$TARGET" in
+  x86_64-unknown-linux-musl) PLATFORM="linux/amd64" ;;
+  aarch64-unknown-linux-musl) PLATFORM="linux/arm64" ;;
+  *)
+    echo "unsupported target: $TARGET" >&2
+    exit 2
+    ;;
+esac
+
+WORKSPACE="$(CDPATH='' cd -- "$(dirname -- "$0")/../.." && pwd)"
+
+# The container appends a [patch.crates-io] section to Cargo.toml and updates
+# Cargo.lock; keep byte-exact copies so the workspace is clean afterwards.
+BACKUP_DIR="$(mktemp -d)"
+cp -p "$WORKSPACE/Cargo.toml" "$BACKUP_DIR/Cargo.toml"
+cp -p "$WORKSPACE/Cargo.lock" "$BACKUP_DIR/Cargo.lock"
+
+restore_manifests() {
+  cp -p "$BACKUP_DIR/Cargo.toml" "$WORKSPACE/Cargo.toml"
+  cp -p "$BACKUP_DIR/Cargo.lock" "$WORKSPACE/Cargo.lock"
+  rm -rf "$BACKUP_DIR"
+}
+trap restore_manifests EXIT
+
+HOST_UID="$(id -u)"
+HOST_GID="$(id -g)"
+export TARGET HOST_UID HOST_GID
+
+docker run --rm \
+  --platform "$PLATFORM" \
+  --env TARGET \
+  --env HOST_UID \
+  --env HOST_GID \
+  --volume "$WORKSPACE:/work" \
+  --workdir /work \
+  "$IMAGE" \
+  sh /work/dist/release/build_musl_container.sh