fix(sparql-anything): remove stale chunks with five or more digits

[ddeboer]

chunk() pads the chunk index to four digits, so the 10,000th chunk onwards gets a five-digit name. removeChunksOf() matched \d{4} exactly and left those behind, so a re-run at a larger chunk size kept the earlier run’s tail in the directory – and in geonames-rdf, in the output.

• Match four or more digits.
• Add a test that chunks a 10,001-row input at one row per chunk, re-chunks at a larger size and checks that only the new chunks remain (one character per row keeps it small; it still writes 10,001 files, so it has a 30 s timeout).
• Docs: take the paths chunk() returns as the list of chunks to convert, rather than listing the directory.

Fix #823