[DRAFT] feature: use schema extensions for JSON and CBOR naming

[jterapin]

Summary

Previous PR added a stopgap reverse map on StructureShape keyed by both location_name and jsonName. That made member resolution protocol-blind: a jsonName wire key could resolve even when the protocol should use the model member name instead. This PR replaces that stopgap with lazy, per-protocol wire-name resolution for JSON and CBOR.

Base branch: serde-data-driven

Description

This PR removes the global reverse-map approach and moves wire-name resolution into the JSON and CBOR serde paths.

Highlights:

• jsonName is now resolved only when the protocol actually uses it.
• JSON and CBOR resolve members through lazy protocol-aware indexes instead of a schema-wide wire-name map.
• MemberShape now uses model_name as the primary modeled member identity.
• Codegen now emits model_name: and symbolized serde trait keys for the JSON/CBOR paths.

Behavior after this change:

• JSON with json_name: true uses jsonName
• JSON with json_name: false uses the model member name
• CBOR uses the model member name

This keeps the fix focused: no Query/XML behavior changes are being introduced here.

Other Notes

• MemberShape#initialize still falls back from model_name: to location_name: for downstream compatibility.
• That fallback is currently needed because some downstream AWS Query/XML wrapper-member paths still construct members with location_name:.
• The old eager member_by_wire_name path is removed.
• The special union __type behavior is preserved only when it is explicitly modeled through jsonName.

Testing

Passed all specs.