Fix Japanese TDT model download filename mismatch (#522)

Fixes the infinite re-download loop for Japanese TDT models reported in
#521.

## Problem
The `download()` function was using hardcoded `Names.decoderFile` and
`Names.jointFile` for all model versions. For `.tdtJa`, this downloaded:
- `Decoder.mlmodelc` 
- `JointDecision.mlmodelc`

But `modelsExist()` checks for version-specific filenames:
- `Decoderv2.mlmodelc`
- `Jointerv2.mlmodelc`

This mismatch caused the existence check to fail, triggering cache purge
and re-download in an infinite loop.

## Solution
Use `getModelFileNames(version)` in the download function to get the
correct filenames for each version, matching what `modelsExist()`
expects.

## Testing
- [x] Build passes
- [x] Filenames now match between download and existence check
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Scripts/parakeet_subset_benchmark.sh

@@ -392,7 +392,7 @@ BASELINE_TDT_CTC_WER="3.6"
 BASELINE_EARNINGS_WER="16.54"
 BASELINE_EOU_WER="7.11"
 BASELINE_NEMOTRON_WER="1.99"
-BASELINE_JA_CER="6.11"
+BASELINE_JA_CER="7.77"
 BASELINE_ZH_CER="8.37"
 
 extract_wer() {