Add TTS audiobook generator with dual-voice effect

Generates Russian audiobooks from ASFR stories using Silero TTS v5.
Narrator voice (eugene) for prose, robotic voice for system errors.

- tts/tts_silas.py: generator script with stress accentor + robotic effect
- tts/README.md: documentation
- .gitignore: exclude output WAV files

tts/README.md

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+# TTS — Audio Generation for ASFR Stories
+
+## What
+
+Generates audiobooks from story files using **Silero TTS v5** (Russian). Two voices:
+- **Narrator** — male voice (`eugene`) for the story prose
+- **Cyborg voice** — same voice with robotic effect (pitch shift + ring modulation + bitcrush) for system error messages (`ОШИБКА:`, `ПРЕДУПРЕЖДЕНИЕ:`)
+
+## Setup
+
+```bash
+uv add torch torchaudio silero silero-stress soundfile librosa
+```
+
+## Usage
+
+```bash
+# Generate Silas audiobook
+uv run tts/tts_silas.py Silas.md
+
+# Generate any story
+uv run tts/tts_silas.py Logan.md
+```
+
+Output goes to `tts/output/`:
+- `<name>_full.wav` — complete audiobook
+- `<name>_demo.wav` — first 4 segments (~1 min)
+
+## Voice Options
+
+Available Silero v5_ru voices:
+- `aidar` — deep male (can be monotone)
+- `eugene` — male, more expressive intonation ← default
+- `baya` — female, soft
+- `kseniya` — female, clear
+- `xenia` — female, calm
+
+Change `SPEAKER` in `tts_silas.py` to switch voices.
+
+## Robotic Effect
+
+System messages get these audio effects:
+- Pitch shift: -4 semitones (deeper, more mechanical)
+- Ring modulation: 50 Hz carrier (metallic timbre)
+- Bitcrush: 200 levels (digital distortion)
+
+Parameters in `make_robotic()` can be tuned.
+
+## Known Issues
+
+- Silero can misplace Russian word stress on complex sentences
+- Long paragraphs are split at sentence boundaries
+- Robotic effect uses librosa which has deprecation warnings (harmless)