Preserve known spend across framework lifecycles

[amavashev]

Summary

• preserve known spend once guarded work or a managed stream has executed
• add caller-scoped settlement idempotency keys and strict post-journal error surfacing
• harden decorator validation and estimate fallback behavior across sync and async paths
• update the LangChain example to use heartbeats, durable recovery, and normalized usage

Verification

• 630 passed, 5 skipped; 98.47% coverage
• Ruff and mypy clean
• sdist and wheel build clean
• shared recovery conformance profile: 12/12 scenarios

Release

Publishes runcycles 0.5.3. The companion LangChain integration PR depends on this release.