Redesign Kompact error handling surfaces

[Bathtor]

Summary

• Add SNAFU-backed opaque public errors for runtime shutdown and network bridge failures, preserving source chains, locations, and optional backtraces in printable diagnostics.
• Redesign serialisation errors around stable actionable kinds and source-preserving variants instead of stringly error payloads.
• Update runtime shutdown flow so timer shutdown still runs after system-component shutdown failure, and add local AGENTS.md SOPs.

Validation

• New runtime error tests cover opaque shutdown display formatting, locations, source chains, and backtrace suffix rendering.
• New serialisation tests cover stable error kinds and preservation of third-party sources.
• New network bridge tests cover exact diagnostic display and direct preservation of send/IO sources.
• Existing workspace unit, integration, doctest, and clippy coverage exercises the migrated public APIs and examples.

Closes #224