spike(identity): prototype cross-server brokerage

[hmans]

Why

Chatto needs a concrete way to evaluate whether accounts on independent servers
can self-verify as the same user without exposing email addresses, trusting a
shared broker, or asking users to manage a persistent identity key. This spike
turns the protocol discussion into an adversarial, executable proof of concept.

What changed

• Add an isolated cli/internal/identitybroker Go package; it is not connected
  to production APIs, persistence, or the frontend.
• Model two-server genesis and later joins requiring a target plus two current
  sponsors on distinct server origins.
• Use origin-pinned Ed25519 server keys, challenge-bound disposable ceremony
  keys, exact length-prefixed canonical signing bytes, genesis-derived group
  IDs, bounded credential lifetimes, and idempotent approval/finalization.
• Make self-revocation atomic with its sole approval and validate the credential
  against verified local group state before committing it.
• Reconstruct the membership DAG and revocations from public certificates on a
  clean client, including an HTTP integration test spanning 20 server origins.
• Document the demonstrated security boundary, the intended RUNTIME_STATE /
  EVT production mapping, and unresolved recovery, rotation, privacy, and API
  questions.

The adversarial review loop fixed discovery-origin poisoning, post-revocation
backdating and selective disclosure, parallel-genesis aliasing, ceremony-key
substitution, unbounded lifetimes, and invalid revocation poisoning. The final
review found no remaining high- or medium-severity issues.

Compatibility and rollout

• No protobuf, public API, persisted event, NATS resource, configuration, or
  user-facing behavior changes.
• The package is deliberately in-memory and must not be used as production
  state. An ADR/FDR and production key/storage design are required before
  integration.

Test plan

• mise test-cli
• mise license-check
• mise x -- go test ./internal/identitybroker -count=1 -timeout 60s
• mise x -- go test -race ./internal/identitybroker -count=1 -timeout 90s
• mise x -- go vet ./internal/identitybroker
• mise x -- go test ./internal/identitybroker -shuffle=on -count=20 -timeout 90s
• mise x -- go test -race ./internal/identitybroker -shuffle=on -count=5 -timeout 120s

Closes #1508.