Model TargetRelations CDC/non-CDC modes with a oneof

Replace the flat relations/inserts/deletes fields on TargetRelations
with a `oneof body { PlainTargets plain; CdcTargets cdc; }`, making the
mutually-exclusive plain (non-CDC) and CDC modes explicit instead of
inferred from which repeated field happens to be populated. PlainTargets
wraps `targets`; CdcTargets wraps `inserts`/`deletes` (oneof can't hold
repeated fields directly, hence the wrapper messages).

The LQP s-expression syntax is unchanged — only the grammar's
construct/deconstruct helpers and the deconstruct guard (now switching on
the oneof case via has_proto_field) change, so pretty/pretty_debug
snapshots are untouched; only .bin wire encodings shift. Regenerated all
three SDKs; updated hand-written Julia equality (PlainTargets/CdcTargets
+ oneof-based TargetRelations), properties global_ids, and equality
tests. All Python/Go/Julia suites pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hbarthels

meta/src/meta/grammar.y

@@ -1153,13 +1153,13 @@ cdc_deletes
 relation_body
     : non_cdc_relations
       construct: $$ = construct_non_cdc_relations($1)
-      deconstruct if builtin.is_empty($$.inserts) and builtin.is_empty($$.deletes):
-        $1: Sequence[logic.TargetRelation] = $$.relations
+      deconstruct if builtin.has_proto_field($$, 'plain'):
+        $1: Sequence[logic.TargetRelation] = $$.plain.targets
     | cdc_inserts cdc_deletes
       construct: $$ = construct_cdc_relations($1, $2)
-      deconstruct if not (builtin.is_empty($$.inserts) and builtin.is_empty($$.deletes)):
-        $1: Sequence[logic.TargetRelation] = $$.inserts
-        $2: Sequence[logic.TargetRelation] = $$.deletes
+      deconstruct if builtin.has_proto_field($$, 'cdc'):
+        $1: Sequence[logic.TargetRelation] = $$.cdc.inserts
+        $2: Sequence[logic.TargetRelation] = $$.cdc.deletes
 
 target_relations
     : "(" "relations" relation_keys relation_body ")"
@@ -1527,13 +1527,11 @@ def _try_extract_value_string_list(value: Optional[logic.Value]) -> Optional[Seq
 
 
 def construct_non_cdc_relations(
-    relations: Sequence[logic.TargetRelation],
+    targets: Sequence[logic.TargetRelation],
 ) -> logic.TargetRelations:
     return logic.TargetRelations(
         keys=list[logic.NamedColumn](),
-        relations=relations,
-        inserts=list[logic.TargetRelation](),
-        deletes=list[logic.TargetRelation](),
+        plain=logic.PlainTargets(targets=targets),
     )
 
 
@@ -1543,22 +1541,17 @@ def construct_cdc_relations(
 ) -> logic.TargetRelations:
     return logic.TargetRelations(
         keys=list[logic.NamedColumn](),
-        relations=list[logic.TargetRelation](),
-        inserts=inserts,
-        deletes=deletes,
+        cdc=logic.CdcTargets(inserts=inserts, deletes=deletes),
     )
 
 
 def construct_relations(
     keys: Sequence[logic.NamedColumn],
     body: logic.TargetRelations,
 ) -> logic.TargetRelations:
-    return logic.TargetRelations(
-        keys=keys,
-        relations=body.relations,
-        inserts=body.inserts,
-        deletes=body.deletes,
-    )
+    if builtin.has_proto_field(body, "plain"):
+        return logic.TargetRelations(keys=keys, plain=body.plain)
+    return logic.TargetRelations(keys=keys, cdc=body.cdc)
 
 
 def construct_csv_data(

proto/relationalai/lqp/v1/logic.proto

@@ -301,15 +301,25 @@ message TargetRelation {
   repeated NamedColumn values = 2; // Value columns for this relation (may be empty)
 }
 
-// Generalized loading: a shared set of key columns and one or more target relations.
-// CDC vs non-CDC is implied by which group is populated:
-//   - `relations` populated => non-CDC outputs
-//   - `inserts`/`deletes`   => CDC insert/delete groups
+// Plain (non-CDC) load: each input row becomes a tuple in every target relation.
+message PlainTargets {
+  repeated TargetRelation targets = 1; // Target relations (each row feeds all of them)
+}
+
+// CDC load: input rows are routed by METADATA$ACTION into insert and delete deltas.
+message CdcTargets {
+  repeated TargetRelation inserts = 1; // INSERT-action rows feed these
+  repeated TargetRelation deletes = 2; // DELETE-action rows feed these
+}
+
+// Generalized loading: shared key columns plus the target relations, loaded either as a
+// plain snapshot or as CDC insert/delete deltas. The two modes are mutually exclusive.
 message TargetRelations {
-  repeated NamedColumn keys = 1;         // Shared key columns (name "METADATA$KEY" => derived hash)
-  repeated TargetRelation relations = 2; // Non-CDC outputs
-  repeated TargetRelation inserts = 3;   // CDC insert group
-  repeated TargetRelation deletes = 4;   // CDC delete group
+  repeated NamedColumn keys = 1; // Shared key columns (name "METADATA$KEY" => derived hash)
+  oneof body {
+    PlainTargets plain = 2;
+    CdcTargets cdc = 3;
+  }
 }
 
 message CSVData {