fspy preload can abort or deadlock cached tasks during signals, fork, or teardown

fspy’s Unix preload must not make a traced process less reliable. Interposed calls can run from signal handlers, post-fork() children, or during teardown, where unsafe runtime state can cause deadlocks or aborts.

Expected behavior:

• Never panic, abort, or deadlock the workload.
• Preserve the original call’s result and errno.
• Work safely during signals, after fork(), and during teardown on Linux and macOS.
• If tracing fails, let the workload continue but skip caching the incomplete trace.
• Add regression coverage for these contexts.

Locate fspy’s Unix preload entry points and follow the cached-task, signal, post-fork, and teardown paths. Check how interposed calls preserve results and errno, then run the existing test suite before adding regression coverage for Linux and macOS in each context. Done means failures skip incomplete caching without aborting, deadlocking, or changing workload results.