Problem:
When `vim.hl.range(0, …, { timeout = N })` is called, the deferred
`range_hl_clear` captures `buf=0`, which resolves to an arbitrary
"current buffer" at timeout. This may cause a stale highlight that never
gets cleared.
Solution:
Resolve `buf=0` explicitly, before `range_hl_clear` captures it.
ec7dab077b