refactor: Drive JSON/CBOR serde off the data with a load-time wire-name map

[jterapin]

Context

V4 walks every declared schema member on each request and response and resolves each wire name inline. This restores the load-time reverse map V3 had and drives JSON/CBOR (de)serialization off the data instead of the full member list.

What changed

• StructureShape builds @members_by_wire_name at add_member, keyed by each member's location_name and jsonName (when different); wire-name collisions raise at load.
• JSON and CBOR structure build/parse now iterate the input values / payload keys and resolve members through the map.
• XML is unchanged; member order is its serialization contract.

Testing

• smithy unit + RBS suites green; aws-sdk-ruby-staging protocol conformance green (awsJson1_0/1_1, restJson1; awsQuery/ec2Query/restXml unaffected).
• Benchmarked vs baseline on real generated clients (stub_responses): codec hot path ~1.5-1.7x faster on wide-sparse structures, ~1.15x wide-dense, batch payloads parity.

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Written with AI assistance and reviewed by jterapin.

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