Simplify block default passing in cattr and mattr

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This is the follow-up promised in #317.

Summary

cattr and mattr split the writers into writers - readers and writers & readers so the default block wouldn't run a second time for an attribute that already has a reader. That split isn't needed. *_reader and *_writer only set the default when class_variable_defined? is false, so once the reader has set it, the writer skips the block anyway.

Both methods now just pass &block through:

cattr_reader(*readers, &block)
cattr_writer(*writers, &block)

The change is in lib/core/facets/module/mattr.rb: 2 lines added, 13 removed.

Testing

I checked cattr and mattr with a Ruby 3.3 script. The #317 behavior is unchanged:

• An existing class variable is kept, not replaced by the default.
• In cattr(:y, :y=) { … } / mattr(:y, :y=) { … } the block runs exactly once.

I didn't run the repo's ruby-test suite locally; CI runs it.

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https://claude.ai/code/session_01E2Njj4j1r9twkVcXsMySLW

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CI note: the only red check here is Ruby jruby on ubuntu, and it isn't caused by this PR.

• It fails in test/standard/test_uri.rb:45 with private method 'hash_to_query' called for module URI (from lib/standard/facets/uri.rb:28). This PR only touches lib/core/facets/module/mattr.rb.
• The JRuby job is marked experimental (continue-on-error), so the workflow run itself passes. All the CRuby versions and TruffleRuby pass.
• The likely cause is alias_module_function :hash_to_query, :query, which is called inside a bare module_function section in uri/query.rb. JRuby 10.1 appears to leave the singleton alias private. I don't have a verified fix yet, so this should be a separate change rather than part of this PR.

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