impl: implement rehearsal writing in `DirectionWriter`

[webern]

DirectionReader::parseRehearsal reads <rehearsal> elements into RehearsalData (text, position, color, enclosure, font) but DirectionWriter has no corresponding write path — rehearsals are silently dropped on output.

RehearsalData already models the full attribute surface (text, positionData, colorData, fontData, enclosure). The writer just needs a loop over directionData.rehearsals that emits <direction-type><rehearsal .../> elements, mirroring the pattern used by segno/coda in PR #203.

PR #134 included a rehearsal writer but was not merged; it was explicitly scoped out of PR #203 to keep that PR focused on segno/coda.

References

• PR api: segno and coda suport #203 (scoped this out; segno/coda writer is a good implementation reference)
• PR Add support for segno reading #134 (previous attempt that included a rehearsal writer)