Why: The Generative UI proposal U2 needs a versioned, host-validated
envelope for agent-authored pages before any backend surface exists:
identity/compat/version envelopes, stable per-node ids with source
annotations, catalog-driven spec validation, and the surface.updated
projection event.
Evidence: go vet + go test in this repo (table tests cover a realistic
valid spec from the U1 trial shape, violations, limits, and old-payload
backward compatibility).
Impact: Additive only - UINode gains optional id/source (omitempty),
old card/form paths unaffected; no existing symbol changes.
Refs: Documents proposal 2026-10-10-generative-ui-plugin.md (r2, U2/U1
sections); Dev task feature/generative-ui-u2
