kern 1.1.1
Concurrent HTTP fix, boxed union layout, release pipeline

Release notes
Concurrent HTTP+Postgres hang
Per-connection HTTP state that was static __thread (WebSocket/SSE/stream flags and the two 16 KB response-header scratch buffers) moved into the per-request context, so a request parking on I/O can no longer have a sibling on the same thread overwrite it.
Boxed union variant layout
A variant box was sized to the raw field sum rather than the aligned payload, so a bool before an 8-byte field left a value read past the allocation.
Linux spawn_thread server
kern_fd_wait takes the libuv park only on the loop thread; worker threads block on poll(2).
Release pipeline
The build image and workflows now install libzstd-dev; without it no v1.1.0 binary assets were ever produced.
New gate
make check-deps-image-parity ensures every enforced deps.lock library has its -dev package in the Dockerfile and release workflows.