Implement voxora-voxtral engine (Mistral Voxtral ASR via Rust binding)

Tracks the planned `voxora-voxtral` engine crate. Per ROADMAP
Phase 7.

## Status

**Blocked on upstream.** Mistral Voxtral (released 2025) has no
Rust binding. The reference implementation is Python (transformers
+ safetensors). A Rust port would either be from scratch, a port
of `transformers` pieces, or a candle-based reimplementation.

## What `voxora-voxtral` will be

A new workspace crate that:

- Implements `voxora_traits::AsrEngine` and
`voxora_engine::EngineAdapter` for Mistral Voxtral.
- Supports `voxora-bridge::EngineFamily::Voxtral`.
- Uses `voxora-hf` for model download.
- Supports `voxora-backend` device selection.

## Upstream blockers

- [Voxtral](https://huggingface.co/mistralai/Voxtral) is published
as safetensors weights. The inference code is in Python
(`transformers` library).
- A candle-based implementation would be the cleanest Rust path
but requires significant work.

## Acceptance

- `voxora-voxtral 0.X.0` published.
- `voxora-bridge::EngineFamily::Voxtral` added.
- Parity test against a real Voxtral checkpoint produces the
expected transcript.

## Out of scope

- Training Voxtral.
- Vision-mode of Voxtral (it can also take images). For now we
target the audio-only ASR use case.