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/event_parser/common/types.rs

@@ -301,7 +301,7 @@ pub struct SwapData {
 pub struct EventMetadata {
     pub signature: Signature,
     pub slot: u64,
-    pub transaction_index: Option<u64>, // 新增：交易在slot中的索引
+    pub tx_index: Option<u64>, // 新增：交易在slot中的索引
     pub block_time: i64,
     pub block_time_ms: i64,
     pub recv_us: i64,
@@ -327,7 +327,7 @@ impl EventMetadata {
         outer_index: i64,
         inner_index: Option<i64>,
         recv_us: i64,
-        transaction_index: Option<u64>,
+        tx_index: Option<u64>,
     ) -> Self {
         Self {
             signature,
@@ -342,7 +342,7 @@ impl EventMetadata {
             swap_data: None,
             outer_index,
             inner_index,
-            transaction_index,
+            tx_index,
         }
     }