Enforce frozen_string_literal and add performance cops

Every file in this project already uses `# frozen_string_literal: true`
but the linter never enforced it, so new files could silently omit the
comment. Enable the Style/FrozenStringLiteralComment cop via standardrb's
extend_config mechanism to make the existing convention a hard rule.

Also add rubocop-performance to catch common performance anti-patterns
like inefficient string operations and unnecessary object allocations.

Author: schneems

.rubocop_extra.yml

@@ -0,0 +1,9 @@
+require:
+  - rubocop-performance
+
+Style/FrozenStringLiteralComment:
+  Enabled: true
+  EnforcedStyle: always
+
+Performance:
+  Enabled: true

.standard.yml

@@ -1 +1,3 @@
 ruby_version: 3.3.0
+extend_config:
+  - .rubocop_extra.yml

Gemfile

@@ -9,6 +9,7 @@ gem "rake", "~> 13.0"
 gem "rspec", "~> 3.0"
 gem "stackprof"
 gem "standard"
+gem "rubocop-performance"
 gem "ruby-prof"
 
 gem "benchmark"