Rebind copied tree ownership to the returned copy

[ac1982]

Copying an element currently copies each attribute's owner pointer, so copied.SelectAttr("p:id").Element() returns the original node. Namespace lookup after editing the copy still consults the original tree, and edits through that owner mutate the original. Document copies also leave top-level tokens attached to a temporary copied container, so removing a copied root does not remove it from the returned document.

Rebind copied attributes to their new element, and rebind the document's direct tokens/attributes after embedding its copied container. Reset an isolated element copy's index to -1, consistent with its nil parent.

Regressions cover element/document copies, copied descendant attributes, namespace changes, edits through Attr.Element(), top-level comments/processing instructions, removal of a copied root, and the detached root index. They fail before their fixes and pass afterward. Full go test ./..., go test -race ./..., go vet ./..., go build ./..., and git diff --check pass on Go 1.27.1 / Linux arm64. Other Go versions/platforms were not run locally.

This is an independent branch from main; it does not include the implicit XML namespace fix in #159.

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