feat: materialize ambient UCP namespaces

[igrigorik]

Implements the reference schema-resolution behavior for Universal-Commerce-Protocol/ucp#657.

Universal-Commerce-Protocol/ucp#657 reserves an ambient ucp protocol namespace at eligible structured object scopes without requiring every open domain source schema to declare it. This change adds a UCP-aware resolution path that makes the namespace explicit in the resolved schema, allowing existing JSON Schema validators and code generators to consume ordinary JSON Schema without implementing a separate payload walk.

Workflow

UCP source schema                bundled ucp.json#/$defs/members
       │                                      │
       └──────────────────┬───────────────────┘
                          ▼
              ambient namespace pass
              • add optional ucp at eligible structured scopes
              • skip dictionary containers
              • preserve explicit ucp visibility
              • keep the namespace open; reject direct ucp.ucp
                          │
                          ▼
              existing operation resolver
              ucp_request / ucp_response
                          │
                          ▼
              ordinary resolved JSON Schema
                          │
                  ┌───────┴────────┐
                  ▼                ▼
          standard validator   schema/codegen tooling

The materializer installs one collision-safe helper under the operation schema's root $defs and uses absolute root-resource references so nested $id scopes and selected $defs continue to resolve correctly. The existing operation resolver remains responsible for directional annotations.

Category

• UCP Schema: Changes to the ucp-schema resolver and validator.

Checklist

• I have followed the contributing guide and Conventional Commits requirements.
• I have added tests covering the new behavior and edge cases.
• New and existing tests pass locally.
• Formatting, linting, and pre-commit checks pass.

Checklist

• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator).
• I have followed the Contributing Guide
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.