feat(shred): tx_index, Migrate without CPI, merge fix, doc limits

- Add tx_index to TransactionWithSlot (entry index), pass to parser
- Emit PumpFun Migrate even when no CPI (shred gets instruction-only)
- Merge: only overwrite Create/CreateV2 with CPI when value non-default
- Rename transaction_index -> tx_index across codebase
- Add docs/SHREDSTREAM_LIMITATIONS.md (ALT/CPI limits, field completeness)
- Comment shred path: static_account_keys only, no inner_instructions

Made-with: Cursor

examples/grpc_example.rs

@@ -113,9 +113,9 @@ async fn test_grpc() -> Result<(), Box<dyn std::error::Error>> {
 fn create_event_callback() -> impl Fn(DexEvent) {
     |event: DexEvent| {
         println!(
-            "🎉 Event received! Type: {:?}, transaction_index: {:?}",
+            "🎉 Event received! Type: {:?}, tx_index: {:?}",
             event.metadata().event_type,
-            event.metadata().transaction_index
+            event.metadata().tx_index
         );
         match event {
             DexEvent::BlockMetaEvent(e) => {