DmfsTask: introduce DmfsTaskFieldBuilder interface + first builder

[rfc2822]

Part of #116. Depends on #338.

Analogous to: events synctools #65

synctools changes

• Create mapping/tasks/builder/DmfsTaskFieldBuilder.kt interface (mirrors AndroidEntityBuilder for events)
  • Core signature: fun build(from: Task, to: ContentValues)
• Create TitleBuilder.kt as the first concrete builder
• Refactor DmfsTaskBuilder to delegate to both the new interface-based builders and the remaining legacy logic
• Add Robolectric-based unit tests per builder class

DAVx5 changes

None needed.