Freeze architecture

## Summary
Based on experiment results, freeze the architecture decisions for the MVP release.

## Success Criteria
- [ ] N-gram order finalized (1/2/3-gram or combination)
- [ ] Confidence threshold finalized
- [ ] Neural model architecture finalized
- [ ] Model size and latency meet requirements
- [ ] Architecture documented in docs/architecture.md
- [ ] No further architectural changes until post-MVP

## Decisions to Finalize
| Component | Options | Decision | Rationale |
|-----------|---------|----------|-----------|
| N-gram order | 1, 2, 3, or combo | | |
| Confidence gate | Fixed threshold, adaptive | | |
| Neural arch | Transformer, LSTM | | |
| Model size | <10MB, <50MB, <100MB | | |

## Freeze Criteria
- Meets accuracy targets from experiments
- Meets latency targets (<100ms p99)
- Meets size constraints (<50MB total)
- Works on target hardware (CPU only)

## Output
- Frozen config file
- Architecture documentation
- Version tag on models