refactor: Rename ShapeRef to MemberShape and add target abstraction for SERDE traversal

[jterapin]

Summary

Renames ShapeRef back to MemberShape and introduces a uniform target abstraction so codecs can traverse member and non-member shapes the same way. Mechanical/structural only — no behavior change. Trait-resolution improvements are deferred to a follow-up PR. Aligns the shape model with Smithy's own vocabulary (the spec's member shape concept and the target key in the Smithy AST

Key changes

Rename + target abstraction

• ShapeRef → MemberShape; .shape/@shape → .target/@target.
• Shape#target returns self, so a bare shape and a MemberShape both respond to
  .target. Codecs dispatch on node.target uniformly — no wrapper needed at the root.

Operation slots are bare shapes

• operation.input, output, and errors now store the targeted StructureShape
  directly (no-input ops resolve to Prelude::Unit) instead of a MemberShape wrapper.
  Consumers read through .target (which is self for a shape), so nothing breaks.

Codec rework (JSON, CBOR, XML)

• Dropped the shape.is_a?(MemberShape) ? shape : MemberShape.new(...) entry wrapper.
• Dispatch on shape.target; renamed the dispatch methods to build_shape/parse_shape
  to align with the Builder/Parser entry points.
• XML DocBuilder output buffer renamed @target → @output (avoids colliding with the
  shape target concept); parser accumulator named result.

Schema codegen view cleanup

• Extracted a shared SchemaHelper (shape-id → constant resolution + prelude map);
  collapsed duplicated build_members; removed dead duplication.

document_utils Serializer / Deserializer

• Same wrapper-drop + serialize_shape/deserialize_shape dispatch.

RBS + repo-wide cleanup

• Updated shapes.rbs (Shape#target, input/output/errors as Shape) and each
  codec's codec.rbs.
• Swept the repo for residual ref/shape_ref/member_ref vocabulary in runtime
  consumers (plugins, handlers, stubbing, param helpers) and specs.

Downstream impact

Once merged, aws-sdk-ruby-staging (version-4) needs corresponding updates: code that
reads operation.input.shape / member.shape must move to .target, and protocol
error-handling that referenced the old accessors needs the .shape → .target change.
Validated against the staging protocol tests during development.

Regenerated

• projections/weather and projections/shapes.