Load yaml only when a YAML layout or object format is used

[tas50]

Problem

logging.rb requires yaml at the top of the file, but YAML is only reachable through two opt-in paths:

• Layout#try_yaml, used when :obj_format is :yaml
• the format method generated by Parseable.create_yaml_format_method, used by Parseable.yaml layouts

Everyone else pays for psych, which drags date and stringio along with it. It is the single largest item in the require graph:

   42.16  logging
   13.19    yaml            <-- largest single item
   12.79      psych
    1.93        date
    5.54    fileutils
    3.53    multi_json
    3.30    logging/appenders
    5.33    logging/layouts

Fix

Move the require into the two funnels.

Note the second one is placed in create_yaml_format_method rather than at the point of use: that method builds a format method whose body calls #to_yaml, so yaml has to be loaded before the generated method runs, not merely before it is defined.

Measurements

Ruby 4.0.6 (arm64-darwin), best of seven runs:

	require 'logging'	files loaded
before	33.21 ms	85
after	21.83 ms	46
	−11.38 ms (34%)	−39

Confirmed directly:

require "logging"                      # yaml loaded? => false
Logging::Layouts::Parseable.yaml       # yaml loaded? => true, layout formats correctly
Logging::Layouts::Basic.new(obj_format: :yaml)  # formats correctly

Two things I looked at and deliberately left alone

• socket (3.59 ms) — Socket.gethostname is evaluated while building the Parseable::DIRECTIVE_TABLE constant, so socket really is needed at load time. Deferring it would mean making the hostname lazy, which is a behavior change, not a require move.
• fileutils (5.54 ms) — logging/utils.rb reopens module FileUtils to add concat. Without fileutils already loaded that would silently define an empty FileUtils module and copy_file/touch would go missing. Not worth the fragility.

Tests

Full suite unchanged: 202 tests, 3 failures, all three pre-existing on Ruby 4.0.6 and unrelated to YAML (two syslog frozen-string-literal warnings, one test_method_name assertion). With the added tests: 206 tests, same 3 failures.

The four added tests cover requiring the gem, a YAML layout, the :yaml object format, and YAML event formatting. The first fails against the previous code.