## Why
The voice helper previously supported only handshake and shutdown, without
validating that its packaged native runtime could be initialized safely.
## What changed
- Add an `initializeRuntime` protocol exchange that loads GStreamer and the
required plugins from physical package paths without opening audio devices.
- Restrict plugin discovery, registry access, and native library search paths,
and keep loaded libraries alive until the helper exits.
- Give initialization a dedicated deadline and terminate the owned helper when
initialization is cancelled or fails.
- Allow binary-only Rust targets to disable the default Bazel library target.
## Testing
- Cover helper-only packages, cancellation, and environment filtering.
- Add an ignored integration test for initialization from a relocated prepared
runtime and rejection of duplicate initialization.
GitOrigin-RevId: 8fff68fe26e52cb2e0722dc98fb5f124eddb0d4d
d979df154c