feat: Add Sherpa ONNX backend for ASR and TTS (#8523)

feat(backend): Add Sherpa ONNX backend and Omnilingual ASR

Adds a new Go backend wrapping sherpa-onnx via purego (no cgo). Same
approach as opus/stablediffusion-ggml/whisper — a thin C shim
(csrc/shim.c + shim.h → libsherpa-shim.so) wraps the bits purego
can't reach directly: nested struct config writes, result-struct field
reads, and the streaming TTS callback trampoline. The Go side uses
opaque uintptr handles and purego.NewCallback for the TTS callback.

Supports:
- VAD via sherpa-onnx's Silero VAD
- Offline ASR: Whisper, Paraformer, SenseVoice, Omnilingual CTC
- Online/streaming ASR: zipformer transducer with endpoint detection
  (AudioTranscriptionStream emits delta events during decode)
- Offline TTS: VITS (LJS, etc.)
- Streaming TTS: sherpa-onnx's callback API → PCM chunks on a channel,
  prefixed by a streaming WAV header

Gallery entries: omnilingual-0.3b-ctc-q8-sherpa (1600-language offline
ASR), streaming-zipformer-en-sherpa (low-latency streaming ASR),
silero-vad-sherpa, vits-ljs-sherpa.

E2E coverage: tests/e2e-backends for offline + streaming ASR,
tests/e2e for the full realtime pipeline (VAD + STT + TTS).

Assisted-by: claude-opus-4-7-1M [Claude Code]

Signed-off-by: Richard Palethorpe <io@richiejp.com>