Problem:
Handlers defer `show()` on unloaded buffers with a buffer-local BufRead
autocmd whose id is stored per-namespace:
- `noautocmd bwipeout` deletes the autocmd, so the next show/hide fails.
- Showing another buffer in the same namespace cancels the pending show.

Solution:
Handlers skip unloaded buffers. The existing global BufRead autocmd
shows cached diagnostics when the buffer loads.

Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

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.

Problem:
`nvim_create_autocmd` is too verbose and its `callback` requires extra
"nesting".

Solution:
Introduce `nvim_on`. Start using it internally. Then we can get a feel
for how it should look before making it public.

Extract the diagnostic implementation from
runtime/lua/vim/diagnostic.lua into focused internal modules covering
config, display, float rendering, jump/list helpers, namespace and
storage management, severity/shared utilities, and statusline support.

Move the builtin handlers into runtime/lua/vim/diagnostic/handlers/ and
keep runtime/lua/vim/diagnostic.lua as the public facade that lazily
dispatches to the split modules. This preserves the external
vim.diagnostic API while making the implementation easier to navigate
and reason about.

AI-assisted: Codex