feat(crypto/playground): payload keyring + key rotation for encrypted payloads

[marcus-pousette]

Summary

This PR adds payload key rotation for encrypted payloads and wires the playground to key IDs (kid) end-to-end.

Crypto

• Add payload keyring model (activeKid + key map) with helpers to create, upsert, and rotate keys.
• Add keyring encryption/decryption APIs:
  • encryptTreecrdtPayloadWithKeyringV1
  • maybeDecryptTreecrdtPayloadWithKeyringV1
  • getTreecrdtEncryptedPayloadKeyIdV1
• Encrypted envelopes now carry a kid, and keyring decrypt requires a matching key id.

Playground integration

• Replace single payload-key storage with per-doc keyring metadata + sealed key entries.
• Invite and grant payloads now include both payloadKeyB64 and payloadKeyKid.
• Add UI to show the active payload key id and rotate payload keys from the auth panel.
• Unknown-key encrypted payloads are surfaced as keyMissing instead of crashing.

Scope

• Remove backward-compatibility shims in this branch (legacy key storage migration and keyring decrypt fallback without kid).

Related: #78