refactor: fix bug in comments right before a Use

[ahomescu]

Root cause (in rewrite/base.rs, not the transform): the
Right/insert branches of rewrite_seq and rewrite_seq_comma_sep anchored
insertions at the neighbors' raw splice_span() boundaries, while the
neighbors' own rewrites and deletions claim their comment-extended spans
(extend_span_comments) — so an insertion anchored at the raw start of a
commented, rewritten item landed inside that item's claimed region and
cleanup_rewrites panicked. The insert branches now extend the neighbor
anchor spans over comments the same way the delete branch always did, and an
insertion at position 0 anchors before the first item's comment-extended span
(the sequence's outer span can start inside it). Inserting exactly at the
extended start of the next item is safe: cleanup_rewrites sorts the
insertion ahead of the following rewrite. The regression test keeps the
comment attached to the import and so covers this fix too.

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