fix: remove deleted benchmark entry and fix clippy doc lints

- Change return type from impl Iterator to Vec<(K,V)> to avoid double-collect
  (previously collected internally then callers collected again)
- Replace coalesce() with simpler dedup_by() for duplicate key handling
- Update all callers to remove redundant .collect() calls

crates/engine/tree/src/tree/payload_validator.rs

@@ -2,9 +2,9 @@
 
 /// Threshold for switching from `extend_ref` loop to `merge_batch` in `merge_overlay_trie_input`.
 ///
-/// Benchmarked crossover: `extend_ref` wins up to ~64 blocks, `merge_batch` wins beyond.
-/// Using 64 as threshold since they're roughly equal there.
-const MERGE_BATCH_THRESHOLD: usize = 64;
+/// Benchmarked crossover: `extend_ref` wins up to ~30 blocks, `merge_batch` wins beyond.
+/// Using 30 as threshold since they're roughly equal there.
+const MERGE_BATCH_THRESHOLD: usize = 30;
 
 use crate::tree::{
     cached_state::CachedStateProvider,