Build a mock host that exercises the cdylib concurrently under sanitizers

[himura467]

Goal

Prove correctness under the conditions the protocol actually imposes: concurrent foreign-thread invocation, strict ownership rules, and memory safety across the ABI.

Tasks

• Rust mock host: dlopens the built cdylib, fires concurrent requests from N OS threads, validates responses, always calls nsgi_free_response exactly once
• Memory/thread sanitizers (ASAN/TSAN, or valgrind on Linux) over the mock-host suite in CI
• Ruby-level unit tests: request wrapping (incl. null-when-len-0 cases), response building, exception→500
• Concurrency stress test proving fibers interleave during I/O (e.g. app that sleeps via the scheduler; total wall time must be ≪ sum of sleeps)
• Buffer-leak test: app stashes an IO::Buffer and touches it after the request completes → must raise, not crash

[himura467]

First cut of the mock host landed at `host/`: it dlopens the cdylib exactly as a real host would, resolves both symbols, and exercises warmup, the app-exception path (must be 500, never a crash), the stashed-buffer path (a buffer held past its request must not read host memory), and 8 threads x 25 concurrent requests with response validation and exactly-once `nsgi_free_response`. Runs clean under macOS `leaks --atExit` (0 leaks). Test app fixture: `host/fixture.rb`.

Remaining: ASAN/TSAN (Linux) in CI and a fiber-interleaving stress test once #4 lands.

[himura467]

feaec63 (PR #9) adds the remaining coverage and CI:

• Fiber-interleaving stress test: 8 threads x 4 sequential 50ms sleeps must finish well under the 1.6s serialized time (asserted by wall clock; ~220ms locally; NSGI_TEST_INTERLEAVE_MS loosens the budget for slow environments).
• Stale-request test: a stashed NSGI::Request must raise NSGI::Error from lazy accessors after its request completed.
• IO::Buffer response body test.
• GitHub Actions (.github/workflows/ci.yml): Linux + macOS matrix building the workspace and running the mock host, plus valgrind memcheck on Linux (leak checking off; the eternal VM's reachable memory is noise) and leaks --atExit on macOS.

Local runs: 239 requests, 0 failures, 0 leaked bytes.