Add rights-transparency discovery members

[miloquinn]

Summary

• Adds operatorName, contactUrl, contentLicense, and rightsStatement as optional discovery-document members (SPECIFICATION.md §3, mirrored in SPECIFICATION.zh-CN.md).
• Types them in schemas/discovery.schema.json.
• Reference server (examples/dart_server.dart) now declares all four; tool/conformance_test.dart asserts they're present with the right shape.
• Purely additive — no existing field, capability, or endpoint meaning changes. No version-number strings were touched; bumping protocolVersion/spec version is left to maintainers per GOVERNANCE.md's release process.

Why

An independent ORSP-compatible reader (Open Reading, github.com/miloquinn/open-reading) already ships these four fields and has for a while, using them purely for client-side transparency display (who operates a source, how to reach them, what license/authorization basis the content is served under). They were never upstreamed, so this spec currently has no normative definition for fields a real-world implementation depends on — anyone else implementing ORSP has no spec to read for this.

Compatibility

• Optional-only: absent fields keep current behavior identical (per Compatibility rules in CONTRIBUTING.md, "Minor releases may add optional fields... only" — this doesn't require a minor bump to merge, since older clients already ignore unknown members per §2).
• No capability token, no new endpoint, no required-field change.
• Client behavior guidance included in the spec text: presence, absence, or content of these fields must not be treated as a legality/authorization/identity certification by any reader, registry, or the protocol itself.

Security / privacy / accessibility / copyright

• No PII collection; these are source-operator-supplied, publicly-displayed strings the operator already controls.
• contactUrl is constrained to absolute HTTP(S), consistent with iconUrl/websiteUrl.
• Encourages copyright/authorization transparency rather than affecting it.

Test plan

• dart format --output=none --set-exit-if-changed examples/dart_server.dart tool/conformance_test.dart
• dart analyze examples/dart_server.dart tool/conformance_test.dart — no issues
• dart run tool/conformance_test.dart — 67 checks passed
• python scripts/validate_contracts.py / registry_tool.py — could not run locally, no Python interpreter available in this environment. Only touched schemas/discovery.schema.json with straightforward additive JSON Schema draft 2020-12 properties, but please let CI confirm.

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