Slice: Add continuum.participant.hello.v1 descriptor fixture #54

Slice

Implement the first protocol-shaped discovery proof from the Continuum Compendium: continuum.participant.hello.v1.

Continuum participant discovery is participant-shaped, not runtime-only. A participant may be a runtime, app, debugger, observer, filesystem membrane, compiler, agent host, warpspace, adapter, fixture runtime, or conformance harness.

Done when:

• descriptor-only participant fixture exists;
• participant kind, supported profiles, contract families, evidence posture, auth posture, redaction posture, and connection hints are represented;
• a redacted or credential-required profile example exists;
• the fixture does not require the participant to pretend to be a runtime.

Part of #59.