Update mansion tax descriptions and methodology

- Add full policy context to map description (bands, £16m estimate, £18.5m PE estimate)
- Update methodology step 4 to explain local property share calculation using NRS dwelling data
- Clarify step 2 wording (identify affected properties vs allocate revenue)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxGhenis