Move `tree/HeavyLightDecomposition` into `datastructures/trees` and remove the stray `tree/` package

Description

There is a top-level tree/ package that contains a single file, HeavyLightDecomposition.java, while every other tree algorithm lives in datastructures/trees/ (41 files).

Having two separate places for tree algorithms is confusing and the lone tree/ package serves no purpose.

Proposed change

• Move HeavyLightDecomposition.java (and its test) into datastructures/trees/.
• Update the package declaration and imports.
• Delete the now-empty tree/ package.

No algorithm behavior changes — this is a structural cleanup.