Replaced the plasma_boundary definition in grammar/vocabularies/locus_registry.yml with the lead-approved text naming the last closed flux surface. The old text named a domain rather than the surface; the new text names the surface itself. Nothing else changed: plasma_boundary_gap, the other locus entries and the conventions concept entry are untouched, and the definition deliberately does not name the separatrix, which the conventions entry forbids for the general case.

Before: The surface used to delimit the plasma domain for the quantity being described.

After: The last closed magnetic flux surface: the outermost flux surface on which field lines close on themselves, bounding the confined plasma.

The concept entry in conventions/concepts.yml does not restate the definition, so the select_conventions accessor joins the locus definition to the conventions equation. Its output for poloidal_flux_at_plasma_boundary is:

token:      plasma_boundary
definition: The last closed magnetic flux surface: the outermost flux surface on which field lines close on themselves, bounding the confined plasma.
phrase:     plasma boundary
equation:   \psi = \psi_{\mathrm{LCFS}}
forbidden:  ['the separatrix']

Measure: the five-file gate ran 39 passed (EXIT=0) at head against 39 passed (EXIT=0) at base — zero added failures. The accessor's pre-change output quoted the old definition, so the comparison distinguishes the two texts.