Problem:
vim.diagnostic.set() defers extmark position computation for an
unloaded buffer via a once=true BufRead autocmd, registering a new one
on every call without replacing the previous one. Each pending autocmd
also retains that call's diagnostics.
Solution:
Instead of registering an autocmd per set() call, register a single
static BufRead autocmd that computes positions from the diagnostic
cache for any buffer with cached diagnostics when it is read. This
removes the per-call registration entirely (nothing left to
accumulate) and means diagnostics cleared while the buffer was
unloaded no longer produce stale extmarks.
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