Remote sessions re-bind the event signer when security tier is raised

What

• Remote-attach sessions' device event signer now supports rebindEventSigner(), which re-reads the session's current device binding and rebinds the signer if it changed, instead of only being set once at attach time.
• The remote session client now handles a new security_update control frame. When it reports security_tier as elevated, the session flags that a raise was seen, and on the next reconnect it calls rebindEventSigner() to re-establish the signer under the new tier.
• Concretely, SessionsV2Client handles this as a new SSE event type carrying security_tier; an elevated tier triggers onSecurityTierRaised, which sets securityTierRaiseSeen and causes the next reconnect to re-key the session's event signer.

Why

This keeps a remote session's cryptographic event signer in sync when the server tightens security requirements mid-session, without forcing the user to start a new session.