Fix #196: honour minChildren when deleting from non-leaf nodes

[pangwangshu]

Summary

• NonLeafHelper.delete previously only dissolved a non-leaf node when it became completely empty, so a node could survive deletion with fewer than minChildren children — violating the R-tree's structural invariant (Honour minChildren with delete operation #196).
• The fix mirrors the existing LeafHelper.delete behavior: when a non-leaf's remaining children drop below minChildren, the node is dissolved and all entries in its subtree (leaf and non-leaf descendants alike) are collected and bubbled up for re-insertion higher in the tree.
• Added getEntries/addEntries helpers to recursively collect entries from a node's children during dissolution.

Test plan

• Added testDeleteIssue196 to RTreeTest, which builds a multi-level tree (maxChildren=3, minChildren=2), deletes entries one at a time, and after each deletion walks the tree verifying every non-root non-leaf node still has at least minChildren children.
• Verified this test fails against the pre-fix code (non-leaf node has 1 children, less than minChildren=2) and passes with the fix.
• Full suite: mvn test — 312 tests, 0 failures, 0 errors.