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

src/streaming/grpc/pool.rs

@@ -276,7 +276,7 @@ impl PooledTransactionPretty {
         let tx = tx_update.transaction.expect("should be defined");
 
         self.transaction.slot = tx_update.slot;
-        self.transaction.transaction_index = Some(tx.index);
+        self.transaction.tx_index = Some(tx.index);
         self.transaction.block_time = block_time;
         self.transaction.block_hash.clear(); // 重置 block_hash
         self.transaction.signature =