Module shift_spans_impls 

ShiftSpans impls for every type reachable from a Directive.

The discipline introduced in round 18 lifted span shifting out of the parser (monolithic shift_directive_inner_spans with per- variant named-field destructure) and into the type system. Round 19 closes the residual hole: enum-variant payloads are now bound BY NAME and dispatched via value.shift_spans(shift) rather than via a wildcard Self::Variant(_) arm. A future payload type change (e.g., wrapping a leaf String payload in Spanned<String>) routes through the wrapper’s ShiftSpans impl automatically — no edit to this file required, no silent discipline gap.

Each type that appears anywhere inside a Directive payload either:

1. Carries no spans — implements ShiftSpans as a no-op via the impl_shift_spans_noop! macro (for foreign leaf types like Decimal, NaiveDate) or via an explicit empty body with exhaustive structural destructure (for variant enums whose payloads currently have no spans, like PriceKind).

2. Has structural payloads — implements ShiftSpans by destructuring every field/variant BY NAME and dispatching into the binding’s own impl. Compound shapes (Vec, Option, Box, Spanned, HashMap) are handled by blanket impls in crate::span and below.

All impls live here (not next to each type) so the discipline is reviewable in one place; a contributor adding a new directive- payload type can use the existing impls as a template.