Add Philippus' solution for day 09 (#922)

Author: Philippus

docs/2025/puzzles/day09.md

@@ -9,5 +9,7 @@ https://adventofcode.com/2025/day/9
 ## Solutions from the community
 - [Solution](https://codeberg.org/nichobi/adventofcode/src/branch/main/2025/09/solution.scala) by [nichobi](https://nichobi.com)
 
+- [Solution](https://github.com/Philippus/adventofcode/blob/main/src/main/scala/adventofcode2025/Day09.scala) by [Philippus Baalman](https://github.com/philippus)
+
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