[Core] Extract ISignatureRenderer (+ constraints/indexers/params)

[jeffpatton1971]

Summary
Move signature/label logic into ISignatureRenderer with SignatureStyle switches (IncludeParamNames, IncludeConstraints, IncludeDefaultValues). Improve indexers and where constraints.

Tasks

• Define interface + default impl; migrate existing logic.
• Extend for indexers and constraints.
• Tests for nested generics and new signature forms.

Acceptance

• Snapshot parity for defaults; new styles covered by tests.