fix(sparql-anything): concatenate outputs without blank lines or listener leaks

[ddeboer]

Two fixes to concatenate() in SparqlAnythingConverter:

• No blank line between outputs. The newline between two per-chunk outputs is now written only when the previous one does not end in one, tracked from the last byte written. SPARQL Anything’s N-Triples writer always ends in a newline, so the unconditional separator put a blank line at every chunk boundary, and the result differed from what cat gives. Consumers that stripped blank lines before diffing against a shell run can drop that.
• No listener leak past ten chunks. Every output ran its own pipeline() into one shared write stream, and each left its listeners on it, so Node printed MaxListenersExceededWarning for runs of more than ten chunks. The outputs now feed one pipeline through an async generator, which also gives a single error path.

Tests cover both: outputs ending in a newline concatenate byte for byte to their joined contents, an output without one still gets a newline before the next, and twelve chunks emit no process warning. The docs section on how a conversion runs describes the new behaviour.

Fix #821