Carry field SourceLocations through the tenant-binding fold rejections

Every rejection the tenant-binding fold produces (TenantBindingIndex: the noTenantBinding family, the node/entity dispatch rejections, and the whole @tenantFanOut ladder) carries SourceLocation.EMPTY, although the SDL field definition’s real location is available at each producing site. The validator therefore prints these author errors without file:line coordinates, unlike the classifier rejections produced during the walk, which makes multi-schema builds needlessly hard to debug. Thread the field definition’s SourceLocation through the fold’s rejection constructors; purely mechanical, no behavioural change.