Lift ColumnField.parentTable from emitter parameter to record component

Surfaced by R237 Phase 2 as a (b-cheap) structural-lift candidate. The classifier produces a ChildField.ColumnField only on a table-backed parent, but the parent table itself is currently threaded into TypeFetcherGenerator.generateTypeSpec as a parameter rather than carried on the ColumnField record. The switch arm at TypeFetcherGenerator.java:319 reads parentTable from the parameter and throws IllegalStateException if null, treating a structurally-precluded reachability as a defensive guard.

Lift: add a non-null parentTable record component to ColumnField, populated at construction in BuildContext.classifyOutputField (the producer site). The compact constructor takes care of non-null enforcement; the generateTypeSpec parameter and the IllegalStateException arm disappear. This lift adds structural type-system enforcement of the table-backed-parent contract, replacing the documentation duplication R237 already removed.

Pre-conditions: none outstanding ; R237 has shipped. The lift stands on its own structural merit.

Out of scope: the multi-record threading on MethodRef.StaticOnly × ReturnTypeRef.TableBoundReturnType (R240’s territory).