[czj]

Every Seahorse::Model::Shapes::Shape and ShapeRef creates an empty metadata Hash in initialize, though most of them never get any metadata. This creates it on the first []= instead, and [] reads a missing Hash as no metadata. It's a small one, next to #3465: about 0.6 MB less per process for S3 and KMS, and the same for every other service API an app loads.

Loading the S3 and KMS APIs (ClientApi::API), on version-3 at 1419898 and with this change, Ruby 4.0.9:

	version-3	this PR
unfrozen empty Hashes left after load	4,186	353
objects allocated while loading	200,783	196,950
ObjectSpace.memsize_of_all	6.06 MB	5.48 MB

Load time doesn't change (within noise, interpreter and YJIT). Query-protocol responses (STS, SQS, SNS, IAM) also allocate 3 Hashes less each, since Query::Handler#rules builds a shape and two refs per response.

Every metadata read on the S3 and KMS models (4,273 shape, ref and operation reads over 27 keys, including one that is never set) gives the same values before and after. aws-sdk-core (2,496 examples) and aws-sdk-s3 (1,721) specs pass, and the new spec covers reading metadata that was never set.

Operation and Api keep their eager Hash, there are only a few of them. Would you like those changed too for consistency, or is it fine to keep this one narrow?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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