Fix STT-only model selection in darkbloom start

[cryptosingheth]

Summary

Fixes #42 for the No models selected loop when selecting only Cohere Transcribe [transcription].

The issue was reproduced on a Mac Mini, but the root cause is not Mac Mini-specific. hardware::detect() only controls which catalog entries fit the machine's RAM; the failure happens later in generic startup selection validation after the picker has already returned selected entries. Any Mac provider that selects only an STT/transcription model can hit the same error.

The separate ModuleNotFoundError: No module named 'mlx_audio.stt.cohere_asr' symptom mentioned in #42 is a runtime/package issue and is not covered by this PR.

Root Cause

cmd_start already split selected picker entries into:

• text models for --model
• image models for --image-model
• transcription/STT models for EIGENINFERENCE_STT_MODEL

However, the final validation only checked text and image selections:

if selected_models.is_empty() && final_image_model.is_none() {
    anyhow::bail!("No models selected");
}

So picked_stt could be populated correctly, but STT-only startup still failed before launchd service installation.

Changes

• Adds a small has_start_selection helper that treats text, image, and STT selections as valid startup workloads.
• Updates the final darkbloom start guard to include picked_stt.
• Prints the selected STT model in the startup summary.
• Adds focused unit tests for text-only, image-only, STT-only, and empty selections.
• Keeps the branch on current upstream master; while validating that base, also applies formatter-only cleanup in crypto.rs / secure_enclave_key.rs and gates the in-process engine scratch variable behind the existing python feature so --no-default-features compiles.

Validation

• cd provider && cargo fmt --check
• cd provider && cargo test --no-default-features

Note: an initial sandboxed test run failed because local mock-server socket binding was denied with Operation not permitted; the same test command passed when rerun outside the sandbox.

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