Problem:
- With `clipboard=unnamed[plus]`, per-cursor yanks are joined into '"'
on exit, but not the clipboard.
- Multicursor `"+yy` calls the clipboard provider per-cursor, behaving
as "last wins".
Solution:
- Never cascade to/from the clipboard provider.
- Implicit clipboard (`clipboard=unnamed[plus]`) uses the cursor-local
unnamed register, so the cascade needs no `start_batch_changes()`. On
exit, the clipboard gets the same joined result as the `"` register.
- Operations that reference the explicit clipboard registers `"+` / `"*`
while multicursor is active, use the cached (primary) clipboard value.
cb2e9cdfd3
