Add missing popular mappers (206, VRC, FME-7, MMC4)

## Summary

Several popular mappers are missing. The highest-impact additions would be:

| Mapper | Name | Games | Notes |
|--------|------|-------|-------|
| **206** | Namcot 108/DxROM | ~44 | 7th most popular mapper. Simplified MMC3 (no IRQ, no mirroring). Straightforward to add based on existing Mapper 4. |
| **10** | MMC4 (FxROM) | ~3 | Similar to MMC2 (Mapper 9). Used by Fire Emblem. |
| **69** | Sunsoft FME-7 | ~10 | Used by Gimmick!, Batman: Return of the Joker |
| **21/23/25** | VRC4 (Konami) | ~16 | Wai Wai World 2, Gradius II |
| **24/26** | VRC6 (Konami) | ~3 | Castlevania III (JP), includes expansion audio |
| **85** | VRC7 (Konami) | ~2 | Lagrange Point (JP), includes expansion audio |
| **19** | Namco 163 | ~17 | Various Namco titles, expansion audio |
| **33/48** | Taito TC0190 | ~14 | Various Taito titles |

### Recommendation

Start with **Mapper 206** — it's the highest impact (most games) and simplest (subset of existing MMC3 code). Then MMC4 (similar to existing MMC2), then FME-7.

## References

- [Mapper list with game counts](https://forums.nesdev.org/viewtopic.php?t=20019)
- [Mapper 206 wiki](https://www.nesdev.org/wiki/INES_Mapper_206)
- [FME-7 wiki](https://www.nesdev.org/wiki/Sunsoft_FME-7)

Start by locating the existing Mapper 4 and Mapper 9 implementations, then compare them with the linked NESDev mapper references. Begin with Mapper 206 as recommended, followed by the other listed mappers if the broader scope is retained. Done means the requested mapper behavior, including expansion audio where applicable, is supported.

Written by the indexing model from the issue text.