Summary

This PR adds Enumerable#join_map method that combines mapping and joining operations in a single pass through the enumerable.

Implements Feature #21386

Motivation

The current pattern of .map { ... }.join(sep) is:

• Repetitive across Ruby codebases
• Inefficient due to intermediate array creation
• Could be more idiomatic

Usage

# Basic usage
[1, 2, 3].join_map(",") { |n| n * 2 }  # => "2,4,6"

# With objects
users = [{name: "Alice"}, {name: "Bob"}]
users.join_map(", ") { |u| u[:name] }  # => "Alice, Bob"

# Default separator is empty string
(1..3).join_map { |n| n }  # => "123"

Performance

Benchmarks show consistent 28-30% performance improvement:

Array Size | map{}.join (ms) | join_map (ms) | Improvement
------------------------------------------------------------
10         |          0.0015 |        0.0012 |       23.2%
100        |          0.0123 |        0.0088 |       28.6%
1000       |          0.1211 |        0.0866 |       28.5%
10000      |          1.2275 |        0.8531 |       30.5%

I haven't committed the benchmark code, but please let me know if I should do that.