Enable EmmyLua's assign-type-mismatch diagnostic after correcting the
runtime annotations and assignments it flags.

Correct option, tuple, and return types, and use separate locals when a
value changes representation. Add guards and narrow casts where needed
so the runtime passes the stricter check. Regenerate the affected docs.

AI-assisted

Enable EmmyLua's need-check-nil diagnostic and resolve nullable values
through more precise types, explicit invariants, and guards for missing
runtime state. Keep analyzer-specific workarounds local and update the
affected API documentation.

Exclude the bundled re.lua from analysis so its nil-check warnings do
not fail CI.

AI-assisted

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

Problem:
on_attach() calls refresh(), but there is no guarantee the attached
buffer is the current buffer. This can make linked editing request
handling assume the wrong window.

Solution:
Call refresh() only if the current buffer is attached. This keeps the
initial highlighting behavior while avoiding making incorrect request.

fix(lsp): define autocmds for capabilities in new(), not on_attach()

Problem: Defining autocmds in on_attach() caused issues when multiple
clients provide the same capability for a buffer. Each attaching client
would "replace" the previously defined one since they are all identical.
Then the first one to detach clears them out and any remaining attached
clients would no longer trigger the autocmd for the capability. Further,
the semantic tokens module itself didn't quite work with multiple
clients since any LspNotify (from any client) would send a token request
to all attached clients with no differentiation and the debounce timer
was shared across all clients.

Solution: Always define the buffer-local autocmds in the capability's
`new()` function, and don't mess with the autocmds in on_attach or
on_detach. The capability framework itself will clear the autocmds when
the last client detaches. Also, refactor a bit of the semantic tokens
module so that the various methods take a specific client_id to perform
the work on, and split out timers so each client has its own.

Problem: linked_editing_range was still doing most of the capability
boilerplate itself.

Solution: refactor it to make use of the common Capability framework for
handling enabling, disabling, etc.

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.