Add TransducerRunner and rename AsrRunner to Seq2SeqRunner (#18961)

Summary:
Pull Request resolved: #18961

Restructure `extension/asr/runner/` to support both Seq2Seq and Transducer ASR architectures:

1. **Rename** `AsrRunner` → `Seq2SeqRunner` (with backward-compat alias in runner.h)
   - `runner.cpp` → `seq2seq_runner.cpp`, `runner.h` → `seq2seq_runner.h`
   - Old `runner.h` kept as a thin include redirect for backward compatibility

2. **Add** `TransducerRunner` for RNN-T/TDT models
   - Extracts the ~190-line greedy decode loop from `examples/models/parakeet/main.cpp`
   - Auto-reads model metadata (blank_id, num_rnn_layers, pred_hidden) from constant_methods
   - Supports both standard RNN-T (duration=1) and TDT (variable durations)
   - Returns `vector<Token>` with frame offsets for downstream timestamp computation
   - Includes `preprocess()` method for models with bundled preprocessor
   - Exposes tokenizer via `tokenizer()` getter for downstream use

3. **Unify Token type**: `parakeet::Token` is now an alias for `asr::Token`

4. **Migrate consumers**
   - Whisper `main.cpp`: `AsrRunner` → `Seq2SeqRunner`
   - Parakeet `main.cpp`: inline decode → `TransducerRunner::transcribe()`
   - Android JNI: updated include (uses backward-compat alias, no API change)

Model-specific post-processing (timestamp computation) remains in `examples/models/parakeet/`.

Differential Revision: D100892465

Author: kirklandsign

examples/models/parakeet/CMakeLists.txt

@@ -75,6 +75,7 @@ endif()
 list(
   APPEND
   link_libraries
+  extension_asr_runner
   extension_llm_runner
   extension_module
   extension_data_loader