Add support for additional runic alphabets

## Summary

Extend runify to support additional runic alphabets beyond Elder and Younger Futhark.

## Runic Alphabets to Consider

### Currently Implemented
- [x] **Elder Futhark** (24 runes, c. 150-700 CE)
- [x] **Younger Futhark** (16 runes, c. 800-1100 CE)

### Proposed Additions

| Alphabet | Runes | Period | Priority | Notes |
|----------|-------|--------|----------|-------|
| **Anglo-Saxon Futhorc** | 28-33 | c. 400-1100 CE | High | Also called Anglo-Frisian runes. Extended to handle Old English phonetics. |
| **Younger Futhark (Long-branch)** | 16 | c. 800-1100 CE | Medium | Danish variant, also used in Norway/Sweden. |
| **Younger Futhark (Short-branch)** | 16 | c. 800-1100 CE | Medium | Rök runes, Swedish-Norwegian variant. |
| **Staveless runes (Hälsinge)** | 16 | c. 800-1100 CE | Low | Simplified variant without vertical staves. |
| **Medieval Futhork** | 16+ | c. 1100-1500 CE | Medium | Latinized Scandinavian runes with dotted variants. |
| **Dalecarlian runes** | ~27 | c. 1500-1800 CE | Low | Late survival in Dalarna, Sweden. Mixed with Latin. |
| **Gothic runes** | 27 | c. 200-500 CE | Low | East Germanic, derived from Greek. Limited attestation. |

## Implementation Notes

- Each alphabet should follow the existing registry pattern (`src/futharks/`)
- Anglo-Saxon Futhorc is the highest priority as it's well-documented and commonly requested
- Younger Futhark variants (long-branch vs short-branch) could be sub-options of the existing `younger` system
- Some alphabets (Gothic, Dalecarlian) have limited scholarly sources and may require more research

## References

- [Wikipedia: Runes](https://en.wikipedia.org/wiki/Runes)
- [Wikipedia: Anglo-Saxon runes](https://en.wikipedia.org/wiki/Anglo-Saxon_runes)
- [Omniglot: Anglo-Saxon Futhorc](https://www.omniglot.com/writing/futhorc.htm)
- [Omniglot: Runic alphabets](https://www.omniglot.com/writing/runic.htm)
- [Uppsala University Runic Forum](https://www.runforum.nordiska.uu.se/en/srd/)

## Acceptance Criteria

- [ ] Add Anglo-Saxon Futhorc with all 33 runes
- [ ] Document each new alphabet in `docs/`
- [ ] Add unit tests for new mappings
- [ ] Update `--list` output to show new systems