Many leading titles plus many surname particles slow parsing quadratically #559

Found by the _pipeline/ sweep for #553.

Symptom. A name with many leading titles and many surname particles takes time proportional to the product of the two counts. Ratio for 4× the input (py3.11, 2026-09-28, best of 5), where 4× is linear:

This has been present since at least 2.2.0.

Mechanism. In _pipeline/_group.py, chain() inside _group_segment evaluates all(is_leading_title(pieces[x], ...) for x in range(k)) at every ambiguous-particle chain site k, inside the while k < len(pieces) loop. The all() short-circuits only at the first non-title, so the cost is (leading titles) × (chain sites). At n=400, cProfile shows about 160,800 generator steps and 162k is_leading_title calls. da Silva behaves the same way.

Fix. Compute the length of the leading-title run once, before the loop, and compare k against it. Check that merge() cannot change that run while the loop is running, or recompute it only where it can. Add a guard row: a _PREFIXED_SHAPES row (#557) or a frame-ratio test, with a recorded negative control.