A particle chain is a surname by construction — Lexicon.particles chains onto the following piece to build a family name. Under the family-first orders the chain is handed whatever position name_order assigns, so it can land in given or in middle:

                     default                        FAMILY_FIRST
Juan de la Vega      family='de la Vega'            given='de la Vega'
Juan Q. de la Vega   middle='Q.', family='de la Vega'   middle='de la Vega'

FAMILY_FIRST_GIVEN_LAST behaves as FAMILY_FIRST does. The grouping is identical in every case — [Juan][de la Vega] — so the chain is built the same way and only its placement differs.

Decision

The particle wins: a chain becomes the family name, whatever order was declared. Particles do not join given names; only titles join to given names.

Mechanism

The chain is built in the grouping stage, before roles exist. assign then hands positions out by _effective_order, and nothing consults the fact that a piece is a particle chain — so under family-first the trailing piece is the given position and the chain goes there.

Scope and verification

Family-first only. The default-order corpus is untouched, so tools/differential should come back with 0 diffs at all three baselines, the same clean result as #361 — that is the property to verify, not a ledger entry.

Related

• A title displaces a particle out of leading position, so Dr. Van Johnson reads as one surname #367 is the same principle violated through given-name titles instead of through name_order (Sir de Mesnil → given='de Mesnil', no surname). That half does move the default-order corpus and needs ledger entries, which is why the two are split.
• Should de Mesnil Juan be all surname, or family de Mesnil plus given Juan? #364 and Mesnil Garcia de strands the particle under FAMILY_FIRST but folds it under FAMILY_FIRST_GIVEN_LAST #365 are not consequences of this: de Mesnil Juan and Mesnil Garcia de have no chain at all, because a leading particle deliberately never chains and a trailing one has nothing to chain onto.
• Supersedes the sentence in particles_ambiguous has no effect on the parse under name_order=FAMILY_FIRST #359 that called Juan de la Vega → given='de la Vega' under FAMILY_FIRST "working as intended". Keep a never-given particle attached under any name_order (#359) #361's code comments describe that row as what the fold's guard mechanically tests rather than as a blessed output, so they stay true either way.