Bridge STT type gaps: surface C++ perf fields in Rust structs

[devin-ai-integration]

Bridge STT type gaps: surface C++ perf fields in Rust structs

Summary

The cactus C++ FFI layer already emits performance/diagnostics fields in its JSON responses (time_to_first_token_ms, decode_tps, ram_usage_mb, token counts, etc.), but the Rust TranscriptionResult (batch) and StreamResult (streaming) structs were silently discarding them during deserialization.

This PR adds the missing fields to both structs so they're available to consumers. All new fields use #[serde(default)], so this is fully backwards-compatible — existing JSON without these fields will deserialize with zero-values.

No downstream consumers are changed yet; this just makes the data available.

Review & Testing Checklist for Human

• Verify u64 vs f64 for token counts: TranscriptionResult uses u64 for prefill_tokens/decode_tokens/total_tokens (C++ writes them as integers), while StreamResult uses f64 (C++ build_stream_response round-trips them through json_number() which returns double). Confirm this matches what each C++ path actually emits in practice — a real transcription run would settle this.
• Run a batch + streaming transcription on macOS and inspect the deserialized structs to confirm the new fields populate with real values (not just zero defaults). This couldn't be tested on the Devin machine since cactus-sys requires ARM.

Notes

• Devin session
• Requested by @yujonglee

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