Fix: handle last root numChildren when NumDocuments is exact multiple of MinKeys

[kael-developer]

Problem

In diskInitBtree(), the last root node's numChildren is computed as:

node.numChildren = int(t.NumDocuments % int64(MinKeys))
if node.numChildren == 0 {
    node.numChildren = MinKeys
}

When NumDocuments is an exact multiple of MinKeys (e.g., 2 × MinKeys), the i+1 == t.NumRoots check applies the "partial last root" adjustment to a root that is actually full. While the == 0 guard sets numChildren = MinKeys, the underlying issue is that the partial-root logic should not apply at all in this case.

Fix

doclite/btree.go: Changed the condition from i+1 == t.NumRoots to i+1 == t.NumRoots && t.NumDocuments%int64(MinKeys) != 0, so the partial-root adjustment only applies when there genuinely is a partial last root.

doclite/btree_test.go: Added TestBtreeDiskInitExactMultiple that inserts exactly 2 × MinKeys documents and verifies:

• All documents are findable via Find()
• The last root's numChildren == MinKeys
• After diskInitBtree() re-initialization, all documents remain findable and numChildren is correct

Testing

go test ./doclite/ -v -run TestBtreeDiskInitExactMultiple
go test ./doclite/ -v