fix: Ensure buffer reset preserves mutability and binary encoding

Replace `buffer = ""` with `buffer = +"".b` so the reset buffer remains
unfrozen and binary-encoded, matching the initial assignment. Without
this, `buffer << chunk` after the reset would raise a FrozenError in
Ruby 3.4+ where string literals are frozen by default.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: keelerm84

lib/ld-eventsource/impl/buffered_line_reader.rb

@@ -66,7 +66,7 @@ def self.lines_from(chunks)
                 end
               end
               if i == buffer.length
-                buffer = ""
+                buffer = +"".b
                 i = 0
               end
               position = i