Implement voxora-granite-speech engine (IBM Granite-Speech ASR via Rust binding)

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

## Status

**Blocked on upstream.** IBM Granite-Speech (released 2024) has no
Rust binding. Reference implementation is Python
(`transformers`). IBM has not published a Rust port.

## What `voxora-granite-speech` will be

A new workspace crate that:

- Implements `voxora_traits::AsrEngine` and
`voxora_engine::EngineAdapter` for IBM Granite-Speech models.
- Supports `voxora-bridge::EngineFamily::GraniteSpeech`.
- Uses `voxora-hf` for model download.
- Supports `voxora-backend` device selection.

## Upstream blockers

- [Granite-Speech](https://huggingface.co/ibm-granite/granite-speech-3.3-8b)
weights are published, but inference is Python-only.

## Acceptance

- `voxora-granite-speech 0.X.0` published.
- `voxora-bridge::EngineFamily::GraniteSpeech` added.
- Parity test passes.

## Out of scope

- Training / fine-tuning Granite-Speech.
- The wider Granite language-model family (text-only).