tr: document how to bench its perf

src/uu/tr/BENCHMARKING.md

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+# Benchmarking `tr`
+
+<!-- spell-checker:ignore aeiou -->
+
+`tr` performance is critical for large data processing pipelines. The implementation uses lookup tables for O(1) character operations.
+
+## Building
+
+```shell
+cargo build --release --package uu_tr
+```
+
+## Basic Benchmarks
+
+### Character Translation
+
+```shell
+# Create test data
+dd if=/dev/zero bs=1M count=10 | tr '\0' 'x' > 10mb_input
+
+# Benchmark translation
+hyperfine --warmup 3 \
+    'tr a b < 10mb_input > /dev/null' \
+    './target/release/tr a b < 10mb_input > /dev/null'
+```
+
+### Character Deletion
+
+```shell
+# Create mixed input
+yes "abcdefghijklmnopqrstuvwxyz" | head -n 1000000 > mixed_input
+
+# Benchmark deletion
+hyperfine --warmup 3 \
+    'tr -d aeiou < mixed_input > /dev/null' \
+    './target/release/tr -d aeiou < mixed_input > /dev/null'
+```
+
+### Character Classes
+
+```shell
+# Benchmark character classes
+hyperfine --warmup 3 \
+    'tr "[:lower:]" "[:upper:]" < mixed_input > /dev/null' \
+    './target/release/tr "[:lower:]" "[:upper:]" < mixed_input > /dev/null'
+```
+
+## Quick Regression Test
+
+```shell
+#!/bin/bash
+cargo build --release --package uu_tr
+
+echo "=== Translation ==="
+hyperfine 'tr a b < 10mb_input > /dev/null' './target/release/tr a b < 10mb_input > /dev/null'
+
+echo "=== Deletion ==="
+hyperfine 'tr -d aeiou < mixed_input > /dev/null' './target/release/tr -d aeiou < mixed_input > /dev/null'
+```
+
+## Performance Notes
+
+- Uses lookup tables instead of hash maps for O(1) operations
+- 32KB I/O buffers for improved throughput
+- Should be competitive with GNU `tr` for most operations
+