Streaming + non-streaming model fail-fast (#442)

Pairing [parakeet] streaming = true with a non-streaming model (e.g. parakeet-tdt-0.6b-v3) used to either fail opaquely on Failed to open tokenizer.model or crash mid-chunk with an ONNX Gather shape error. Now the streaming pipeline checks the registry before loading and bails out with a message that names the model and the streaming-compatible alternative.

Unit test

cargo test --features parakeet --lib \
  transcribe::parakeet_streaming::tests::new_rejects_streaming_on_known_incompatible_model \
  -- --nocapture
# Expected: passes; error message names the model and the recommended replacement.

Structural verification

grep -c "is_known_parakeet_model\|is_streaming_compatible_parakeet" \
  src/setup/model.rs src/transcribe/parakeet_streaming.rs
# Expected: 4+ (both helpers plus their call sites)

Runtime test (requires --features parakeet)

1. Set config to a non-streaming Parakeet model with streaming on:

   cat <<EOF >> ~/.config/voxtype/config.toml
   engine = "parakeet"
   [parakeet]
   model = "parakeet-tdt-0.6b-v3"
   streaming = true
   EOF

2. Restart the daemon and inspect the journal:

   systemctl --user restart voxtype
   journalctl --user -u voxtype --since "30 seconds ago" | grep -A8 "Parakeet streaming"

   Expected error body:

   Parakeet streaming is enabled but model `parakeet-tdt-0.6b-v3` does not
   support cache-aware streaming.
   Fix one of:
     - Disable streaming in config.toml under [parakeet]:
         streaming = false
     - Or switch to the streaming-compatible model:
         voxtype setup model parakeet-unified-en-0.6b
     and set [parakeet] model = "parakeet-unified-en-0.6b" in config.toml.