Drop METADATA$KEY special-casing from LQP comments

On the LQP side a key column name is just a string — there is no special
handling of METADATA$KEY (the row-hash derivation happens downstream in
the loader). Remove the "special name METADATA$KEY => derived hash"
wording from the NamedColumn.name and TargetRelations.keys proto comments
(regenerated the Go binding), and reword the analogous comment in the
relations_cdc fixture. The METADATA$KEY column-name strings in fixtures
are real data and stay.

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

Author: hbarthels

proto/relationalai/lqp/v1/logic.proto

@@ -291,7 +291,7 @@ message StorageIntegration {
 // A single named column with its type. Used to describe both shared key columns and
 // per-relation value columns in the generalized `TargetRelations` loading construct.
 message NamedColumn {
-  string name = 1; // Column name (e.g. "src"); special name "METADATA$KEY" => derived hash
+  string name = 1; // Column name (e.g. "src")
   Type type = 2;   // Column type
 }
 
@@ -315,7 +315,7 @@ message CDCTargets {
 // 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 NamedColumn keys = 1; // Shared key columns
   oneof body {
     PlainTargets plain = 2;
     CDCTargets cdc = 3;

sdks/go/src/lqp/v1/logic.pb.go

@@ -4,8 +4,7 @@
       (define
         (fragment :f1
           ;; Generalized CSV loading with CDC: a single `edge` relation grouped into insert
-          ;; and delete deltas. The shared key is the special METADATA$KEY (a UINT128 hash
-          ;; derived from the CSV's METADATA$ROW_ID column).
+          ;; and delete deltas under a shared UINT128 key column.
           (csv_data
             (csv_locator
               (paths "s3://bucket/edges.csv"))