Remove unused index variable from producer-consumer queue

The index was being created, enqueued, and then ignored in the writer.
There's no ordering logic that uses it - the queue itself maintains
FIFO order.

Changes:
- Remove `each_with_index` in build_producer_tasks (use `map`)
- Enqueue chunks directly instead of `[idx, chunk]` pairs
- Remove destructuring in build_writer_task
- Simplify variable names (pair -> chunk, item -> chunk)

Benefits:
- Cleaner code
- Slight performance improvement (no array allocation per chunk)
- Removes misleading code (index suggested ordering logic that didn't exist)

Verified: All 21 tests in stream_spec.rb still pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ihabadham

react_on_rails_pro/lib/react_on_rails_pro/concerns/stream.rb

@@ -91,7 +91,7 @@ def drain_streams_concurrently
     end
 
     def build_producer_tasks(parent:, queue:)
-      @rorp_rendering_fibers.each_with_index.map do |fiber, idx|
+      @rorp_rendering_fibers.map do |fiber|
         parent.async do
           loop do
             # Check if client disconnected before expensive operations
@@ -101,7 +101,7 @@ def build_producer_tasks(parent:, queue:)
             break unless chunk
 
             # Will be blocked if the queue is full until a chunk is dequeued
-            queue.enqueue([idx, chunk])
+            queue.enqueue(chunk)
           end
         rescue IOError, Errno::EPIPE
           # Client disconnected - stop producing
@@ -113,11 +113,10 @@ def build_producer_tasks(parent:, queue:)
     def build_writer_task(parent:, queue:)
       parent.async do
         loop do
-          pair = queue.dequeue
-          break if pair.nil?
+          chunk = queue.dequeue
+          break if chunk.nil?
 
-          _idx_from_queue, item = pair
-          response.stream.write(item)
+          response.stream.write(chunk)
         end
       rescue IOError, Errno::EPIPE
         # Client disconnected - stop writing