The piece tree editor was written against an AvlTree where the caller
owned the node layout. It now runs on the shared AvlTree<Key, Data>:
PieceKey is the key and keeps the piece span plus the cached spans of
both subtrees, recomputed in updateNodeCache from the node's Piece.
Lookup by text position descends with keyInRightSubtree, so the tree
again uses relative keys in findSubTree.
AvlTree gains positional operations needed for a sequence tree whose
keys are implicit: insertAfter, removeNode, refreshNode and next/prev
in-order traversal. The editor uses these instead of a hand-kept
linked list and insertion flags.
The module lives in TextEditor/ with its own library and a UnitTest++
suite, including a randomized comparison against std::string.
Strings/tests/tests.cpp came from the old branch and included a header
that no longer exists, so it is removed.
f24662d1a1