v0.3.1 — campus.common.protocol

New campus.common.protocol module: wire-protocol constants (journey/device cookie names + TTLs; trace/journey/device header names) as fixed, deployment-invariant vocabulary (campus#1041). Single source of truth for header names formerly mirrored in campus.flask_campus.propagation, campus.audit.middleware.tracing, and campus_python.tracing. Additive (patch); lean-import contract untouched.