Link toggle rule refinement (on top of 1ad2d71c8): the toggle now
unlinks only when the selection is FULLY linked internally — the C++
crude "any member has ANY link" check made split halves (which inherit
the original clip's A/V links via BlockSplitPreservingLinksCommand)
impossible to link to each other. Test: link_unlink_toggles_the_
selection_links covers toggle/undo/split-link.
Node editor: only stream inputs (texture/samples) become ports — OFX
plugins declare every parameter as an input, which buried the card
under internal params (the garbled NatronOfxParamProcess* wall). Secret
inputs stay hidden; port labels use the input's display name (the OFX
param label); edges resolve ports by input id, not by label.