docs: add comprehensive group_by aggregation examples

- Document group_by().agg(u256.sum()) use case from Polars issue #15443
- Add practical blockchain transaction aggregation examples
- Show mixed aggregations with regular Polars operations
- Include working example with large number handling

This addresses the core DeFi/blockchain analytics use cases discussed
in the GitHub issue.

Author: elyase

README.md

@@ -127,6 +127,30 @@ u256.to_hex(col)            # → hex strings (0x...)
 u256.to_int(col)            # → Python int (if fits in i64)
 ```
 
+### Group-By Aggregations
+```python
+df.group_by("account_id").agg(
+    u256.sum(u256.from_int(pl.col("tx_amount"))).alias("total_spent")
+)
+
+# Multiple aggregations
+df.group_by("token_address").agg([
+    u256.sum(u256.from_int(pl.col("balance"))).alias("total_supply"),
+    pl.len().alias("holder_count"),
+    pl.col("last_updated").max()
+])
+
+# Mixed with regular Polars aggregations
+df.group_by("wallet").agg([
+    u256.sum(u256.from_int(pl.col("wei_balance"))).alias("total_wei"),
+    pl.col("gas_used").sum(),
+    pl.col("block_number").max()
+]).with_columns(
+    # Convert to readable hex for display
+    u256.to_hex(pl.col("total_wei")).alias("total_wei_hex")
+)
+```
+
 ### Display Utilities
 ```python
 u256.format_u256_dataframe(df, cols)    # Format u256 columns as hex

examples/groupby_example.py

@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+"""
+Group-by aggregation example - demonstrates the exact use case from Polars issue #15443
+
+This example shows how to aggregate U256 values by groups, which is essential for:
+- Blockchain transaction analysis (total spend per account)
+- Token balance aggregation (total supply per token)  
+- DeFi protocol analytics (TVL per pool)
+"""
+
+import polars as pl
+import polars_u256_plugin as u256
+
+def main():
+    print("🔗 Blockchain Transaction Aggregation Example")
+    print("=" * 50)
+    
+    # Simulate blockchain transaction data
+    transactions = pl.DataFrame({
+        "account_id": ["alice", "bob", "alice", "charlie", "bob", "alice", "charlie"],
+        "tx_amount_wei": [
+            1_000_000_000_000_000_000,    # 1 ETH in wei
+            2_500_000_000_000_000_000,    # 2.5 ETH  
+            500_000_000_000_000_000,     # 0.5 ETH
+            10_000_000_000_000_000_000,   # 10 ETH
+            1_200_000_000_000_000_000,    # 1.2 ETH
+            750_000_000_000_000_000,     # 0.75 ETH
+            3_300_000_000_000_000_000,    # 3.3 ETH
+        ],
+        "gas_used": [21000, 45000, 21000, 85000, 32000, 21000, 52000],
+        "block_number": [18500000, 18500001, 18500002, 18500003, 18500004, 18500005, 18500006]
+    })
+    
+    print("📊 Raw transaction data:")
+    print(transactions)
+    
+    # Aggregate by account - the core use case from GitHub issue #15443
+    print("\n💰 Total spending per account (U256 aggregation):")
+    account_totals = transactions.group_by("account_id").agg([
+        u256.sum(u256.from_int(pl.col("tx_amount_wei"))).alias("total_wei"),
+        pl.col("gas_used").sum().alias("total_gas"),
+        pl.len().alias("tx_count"),
+        pl.col("block_number").min().alias("first_block"),
+        pl.col("block_number").max().alias("last_block")
+    ]).with_columns(
+        # Add human-readable ETH amounts
+        (u256.to_int(pl.col("total_wei")) / 1_000_000_000_000_000_000).alias("total_eth"),
+        u256.to_hex(pl.col("total_wei")).alias("total_wei_hex")
+    ).sort("total_eth", descending=True)
+    
+    print(account_totals.select(["account_id", "total_eth", "tx_count", "total_gas"]))
+    
+    # Show hex representation for verification
+    print("\n🔍 Detailed breakdown with hex values:")
+    for row in account_totals.iter_rows(named=True):
+        print(f"{row['account_id']:8s}: {row['total_eth']:6.2f} ETH "
+              f"({row['tx_count']} txs, {row['total_gas']:,} gas)")
+        print(f"         Wei: {row['total_wei_hex']}")
+    
+    # Demonstrate large number handling with hex input
+    print(f"\n🚀 Handling very large numbers (from hex):")
+    large_amounts = pl.DataFrame({
+        "protocol": ["uniswap", "compound", "aave"],
+        "tvl_hex": [
+            hex(2**200),  # Astronomically large TVL as hex
+            hex(2**180),  
+            hex(2**190)
+        ]
+    })
+    
+    protocol_total = large_amounts.select(
+        u256.sum(u256.from_hex(pl.col("tvl_hex"))).alias("total_tvl")
+    ).with_columns(
+        u256.to_hex(pl.col("total_tvl")).alias("total_tvl_hex")
+    )
+    
+    print("Total Protocol TVL (impossible with standard int types):")
+    print(f"Hex: {protocol_total[0, 'total_tvl_hex']}")
+    
+    print("\n✅ All operations completed successfully!")
+    print("🎯 This demonstrates the exact group_by().agg(u256.sum()) use case")
+    print("   mentioned in Polars issue #15443")
+
+if __name__ == "__main__":
+    main()