vim.diagnostic.Opts.VirtualText has a `source` option to prefix
diagnostic messages with their source (`'if_many'` or always), but the
newer virtual_lines handler never got the same option, even though the
shared helpers it needs (diagnostic_shared.prefix_source/count_sources)
are already render-target-agnostic and used by virtual_text as-is.

Add the same `source` field to vim.diagnostic.Opts.VirtualLines and
apply it in the virtual_lines handler right after message formatting,
mirroring virtual_text's own ordering (format first, then prefix the
source onto the already-formatted message).

AI-assisted

Problem: when the highlighting surrounding the diagnostics section is
not StatusLine[NC], the in-section highlights are not combined and not
correctly reset afterwards.

The spaces between the diagnostic items have the highlight of the item
to the left, which looks bad when the Diagnostic* highlight groups are
customised with background colour or underlines.

Solution: use %$ to combine, and %#(...%#) to reset.

ref #36685
ref #35806

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

Quickfix entries can omit their start and end positions. Default absent
start coordinates to the first position and absent end coordinates to
the start, avoiding arithmetic and comparisons on nil. Cover both cases
in the conversion tests.

AI-assisted

- call-non-callable
- missing-fields
- redefined-label
- redefined-local
- undefined-field
- unreachable-code

Promote redefined-local and unreachable-code to warnings.

Fix DOS line-ending handling for multiline semantic tokens, and support
intersection types in the help parser.

AI-assisted