fix(tree): update rendered node values in SetValue

[jakezwang]

tree.Node.SetValue updates GivenValue, but leaves the underlying rendered tree value unchanged. Value and String still show the old label, and a child node keeps the old label even after the model reloads its nodes.

Update the underlying tree value in the setter as well. Tests cover string, numeric and fmt.Stringer values, preserving child content, and rendering a changed child through the public tree model.

Validation: the focused regression and complete test suite pass with -race on Go 1.27.1; the full suite also passes on Go 1.26.6. go build ./... and the CI-pinned golangci-lint 2.13.1 pass.