feat(core): handle RelCommon data for every relation type

AGENTS.md

@@ -114,6 +114,19 @@ Proto conversion is split into two directions, and the class name tells you whic
   to one proto value, order the deprecated ones **last** so the reverse lookup returns the
   canonical one (see `Join.JoinType`). Adding an enum value is an edit to the enum, not the
   converters.
+- **`RelCommon` data** (emit mapping, hint, rel anchor, common advanced extension) is converted by
+  one helper per direction: `ProtoRelConverter.applyRelCommon(rel, relCommon)` on the way in and
+  `RelProtoConverter.common(Rel)` on the way out. Both are still *called* per relation, so both
+  halves are easy to forget: a new `newXxx` must route its result through `applyRelCommon`, and a
+  new `visit` must call `.setCommon(common(rel))`. `RelCommonRoundtripTest` covers every
+  `RelVisitor` type and fails when one has no sample — but it is keyed on POJO types, not on proto
+  `oneof` cases, so a new case mapping to an existing POJO type slips through. `applyRelCommon`
+  runs *after* `build()`, so a relation with a `@Value.Check` on one of these fields must also set
+  it on its builder (see `newLateralJoin`). `UpdateRel` is the only *modeled* relation message with
+  no `common` field (`ReferenceRel` has none either, but no POJO models it); the POJO accessors live
+  on `Rel`, so `NamedUpdate` can still *hold* this data —
+  and an emit mapping would change its `getRecordType()` — so `RelProtoConverter.checkNoRelCommon`
+  rejects it on the way out rather than silently serializing a plan with a different schema.
 - POJO types are created with `TypeCreator.REQUIRED` / `TypeCreator.NULLABLE`.
 
 ## Building and testing

core/src/main/java/io/substrait/dsl/SubstraitBuilder.java

@@ -822,6 +822,14 @@ public NamedUpdate namedUpdate(
   /**
    * Creates a named update relation that updates rows in a table with output field remapping.
    *
+   * <p>Note that {@code UpdateRel} has no {@code common} field as of spec v0.99.0, so it has
+   * nowhere to carry the emit mapping. The resulting relation therefore cannot be serialized:
+   * {@link io.substrait.relation.RelProtoConverter} rejects it rather than emitting a plan whose
+   * schema silently differs from the relation's own {@link
+   * io.substrait.relation.Rel#getRecordType()}. Until the spec gives {@code UpdateRel} a {@code
+   * common} field, use {@link #namedUpdate(Iterable, Iterable, List, Expression, boolean)} for
+   * relations that have to survive a round trip.
+   *
    * @param tableName the qualified name of the table to update
    * @param columnNames the names of the columns in the table
    * @param transformations the list of transformation expressions to apply