Add implementation details for inline errors

doc/developer/design/20240609_error_handling.md

@@ -187,7 +187,60 @@ described in [#6367].
 
 ### Implementation
 
-TODO.
+#### Inline errors
+
+When `VALUE DECODING ERRORS = INLINE` is set:
+
+[`KafkaSourceConnection` `metadata_columns`](/src/storage-types/src/sources/kafka.rs#L59)
+will include a new `error` column in [`plan_create_source`](/src/sql/src/plan/statement/ddl.rs#L605).
+
+[`UpsertStyle`](/src/storage-types/src/sources/envelope.rs#L142) will be extended with a new
+`ValueErrInline` enum value to indicate the inline style should be used.
+```
+#[derive(Arbitrary, Clone, Debug, Serialize, Deserialize, Eq, PartialEq)]
+pub enum UpsertStyle {
+    /// `ENVELOPE UPSERT`, where the key shape depends on the independent
+    /// `KeyEnvelope`
+    Default(KeyEnvelope),
+    /// `ENVELOPE DEBEZIUM UPSERT`
+    Debezium { after_idx: usize },
+    /// `ENVELOPE UPSERT`, where any decoded value will get packed into a ScalarType::Record
+    ///  named `value`, and any decode errors will get serialized into a ScalarType::Record
+    /// named `error`. The error will be propagated to the error stream if `propagate_errors`
+    /// is set. The key shape depends on the independent `KeyEnvelope`.
+    ValueErrInline {
+      key_envelope: KeyEnvelope,
+      propagate_errors: bool,
+    },
+}
+```
+and this will be set in [`plan_create_source`](/src/sql/src/plan/statement/ddl.rs#L605) based
+on the value of the `VALUE DECODING ERRORS` option. If `PROPAGATE` is also included in the
+option value, `propagate_errors` will be set to true.
+
+[`UnplannedSourceEnvelope::desc`](/src/storage-types/src/sources/envelope.rs#L79) will be
+updated to handle the new `UpsertStyle::ValueErrInline` value, with the same logic as
+`UpsertStyle::Default` to determine the column-key and key_desc but returning a
+`ScalarType::Record` in a `value` column rather than merging the value desc into the
+top-level desc.
+
+The source rendering [`upsert_commands`](/src/storage/src/render/sources.rs#L520) method
+will be updated to handle the new `UpsertStyle::ValueErrInline` style. If it receives a
+`DecodeError` row it will serialize the error into a `record(description: text, code: text)`
+and include that in the `error` column, and if it receives a valid value Row it will insert
+the value row into a `record` datum for the `value` column.
+
+If `propagate_errors` is set to `true`, it will continue to produce an additional row with the
+`UpsertValueError` error. This will require switching the `map` collection operator to a `flat_map`. 
+
+At this point the downstream upsert operators should not require any additional changes, as they
+will continue to operate on the same
+`Collection<G, (UpsertKey, Option<Result<Row, UpsertError>>, FromTime), Diff>` input collection
+received from `upsert_commands` as before.
+
+#### Dead-letter queue (DLQ)
+
+TODO
 
 **Open questions:**