fix: validate and settle deferred actor replies

[cevr]

Summary

Validate deferred actor replies against the declared reply schema.
Keep the waiting caller from hanging when state exit interrupts decoding.

Changes

• Keep the reply schema with the pending request.
• Decode before reply completion. Preserve decode defects and interruption causes.
• Add tests for decoded values, invalid values, repeated replies, postponed requests, and shutdown.
• Document the deferred reply contract and add a patch Changeset.

Test Plan

• Full gate: 354 tests passed. Typecheck, lint, formatting, build, and examples passed.
• Independent focused check: reply and eager Effect setup tests passed.
• Gated decoder test: state exit interrupts decoding, settles the caller, and leaves the actor available.

Prepared with Codex.