Refactor aggregate strategies and add a Grouped strategy

README.md

@@ -23,3 +23,23 @@ def deps do
   ]
 end
 ```
+
+## Aggregate Strategies
+
+`AshSql.Implementation` defaults aggregate planning to `:lateral`. SQL data
+layers can override `aggregate_strategy/1` with `:grouped` when they need the
+SQLite-style grouped aggregate implementation.
+
+The grouped strategy uses adapter-provided list aggregation. Implementations
+that select `:grouped` must implement `grouped_list_aggregate/2` and return a
+list expression over the `:ash_sql_grouped_aggregate_window` window. The
+grouped strategy assumes SQLite-compatible SQL: list defaults are JSON-encoded,
+so list values must use a JSON list representation, and offset-only query
+aggregates use `LIMIT -1`. AshSQLite uses SQLite's JSON list representation
+for this callback.
+
+The aggregate facade normalizes resource aggregates and SQL aliases before
+dispatch. Both strategies preserve the source binding and attachment path, so
+aggregates referenced through joined relationships attach to the related row.
+Alias reuse is scoped to that path and follows the existing filter/sort identity
+rules. Public aggregate names, including strings, are retained in results.