refactor(try): parallel try-block dispatch with flat-buf slot tracking

All <esi:try> includes are dispatched into the same select() pool as bare
includes, removing the Nx sequential penalty for consecutive try blocks.
Try-block content reuses the main buf slots (no separate content_slots);
a TryBlockTracker/AttemptTracker pair records which slots belong to each
attempt and assembles the result when all resolve.

- 3 new types: TryBlockTracker, AttemptTracker, SlotEntry
- Request correlation keyed by (method, URL)
- process_queue renamed from process_ready_queue_items for clarity

Author: vagetman

esi/src/element_handler.rs

@@ -88,7 +88,7 @@ pub(crate) trait ElementHandler {
     ///
     /// Called after processing each top-level element in the main parse loop.
     /// Default is a no-op — only meaningful in the streaming context.
-    fn process_ready_queue_items(&mut self) -> Result<()> {
+    fn process_queue(&mut self) -> Result<()> {
         Ok(())
     }