Implement `solution(files, mode)`, a file-deduplication pipeline that finds files with identical content and, in `link` mode, reports the hard-link operations to deduplicate them. **Input.** `files` is a list of 4-tuples `(path, device_id, inode_id, chunks)`. `path` is unique; `device_id` is the file's device; `inode_id` is its inode (files sharing an `inode_id` are already hard-linked); `chunks` is a list of strings whose concatenation in order is the content (chunks may be empty; the list may be empty). `mode` is `'report'` or `'link'`. Walk the chunks directly; never concatenate a whole file into one big string. **Duplicates.** Two files are duplicates when their reconstructed content is exactly equal (chunk boundaries don't matter: `['hello','world']` equals `['helloworld']`). A duplicate **set** is a maximal collection of 2+ content-equal files; unmatched files aren't reported. **Output.** Return a 2-tuple `(groups, links)`. `groups`: a list of duplicate sets; each set is its paths sorted lexicographically; only sets of 2+ are included; sets are ordered by their smallest (first) path. `links`: `[]` in `'report'` mode. In `'link'` mode, per set, handle each device separately: links are valid only within the same `device_id`. On a device, the lexicographically smallest path among that set's files there is the canonical target; for every other file on that device, emit `(path, canonical_path)` only if its `inode_id` differs from the canonical's. Sort the final `links` ascending. **Constraints.** `0 <= len(files) <= 200000`; `0 <= total characters across all chunks <= 1000000`; each `path` is unique; `mode` is `'report'` or `'link'`. **Example.** `[('/a',1,100,['hello','world']),('/b',1,101,['helloworld']),('/c',2,200,['hello','world']),('/d',1,102,['hello','there'])]` in `'link'` mode returns `([['/a','/b','/c']], [('/b','/a')])`.