Where does Freiherr St John Smith MA start: should a title that is also a particle open the name?

freiherr and st are both titles and particles (TITLES ∩ particles_ambiguous). Where one stands second at the head, three places each decide where the name starts:

leading_titles (the title run);
_chain_units' unit opener inside the titles (Should S2's trailing read count the particle chain's units instead of merging a copy of them? #620);
_group's chain leading scan.

#614's index defect lived exactly there, and #620's review found a second case: a unit opened inside the titles whose words the read then takes, which the chain never builds (St St VI, fixed with a gate). Today:

Input	Today
`Freiherr St John Smith MA`	title 'Freiherr', family 'St John Smith', suffix 'MA', with a `title-or-name` report saying "read as a given name" about the family
`Freiherr St van Berg MA`	title 'Freiherr', family 'St van Berg', suffix 'MA'
`Dr. St John`	title 'Dr. St', family 'John'

Proposal: decide the name start once, before the read, and have the read, the chain and assign take that one answer, so the gate in _chain_units (q > k + 1) and assign's re-read fallback can go.

Measure: grids with both title-particles in every head position (AGENTS.md's sweep paragraph); expect moves only in Freiherr/St heads.