test(plugin-sdk): add integration test loading a panicking dylib

[staging-devin-ai-integration]

Context

Follow-up from PR #353 review (FFI panic boundaries).

Problem

The panic guards added in PR #353 have unit tests verifying each guard catches panics and returns correct sentinel values, but there is no integration test that actually loads a .so plugin which panics and asserts that the pipeline survives.

Suggested approach

1. Create a small test plugin crate (e.g. tests/fixtures/panicking-plugin/) that panics in process_packet().
2. Build it as a .so during cargo test.
3. Load it via NativeNodeWrapper, send a packet, and assert the host receives an error (not an abort).

Labels

testing, plugin-sdk