Fix era mixing: sanitise legacy preset refs in grafted templates

Templates harvested from pre-Live-11 projects carry old-style FileRefs
(RelativePath with RelativePathElement children, no Value attribute)
inside LastPresetRef / PresetRef. Splicing such a node into a Live 12
set makes the file unloadable: 'Required attribute Value missing'.
These refs are cosmetic browser pointers, not plugin state:

- _strip_preset_refs reduces them to the modern empty forms
  (<LastPresetRef><Value/></> and <PresetRef/>) on every grafted node
  (AU replacements, VST3 PluginDesc + SourceContext grafts).
- Writer now also gates on the document's RelativePathElement count not
  growing - ET parses old-era XML happily, so schema validity alone
  cannot catch era mixing.