Avoid fspy wait hangs from detached senders

[wyf027]

Summary

• avoid blocking forever when fspy's IPC receiver cannot acquire the lock after the root child exits because a detached descendant still owns a sender
• mark that run's path tracking as incomplete so vite-task skips cache updates instead of storing an unsafe fingerprint
• add a regression test for a shell command that backgrounds a descendant process

Fixes #396

Tests

• cargo fmt --all --check
• cargo test -p fspy_shared ipc::channel::tests::forbid_new_senders_after_locked
• cargo test -p fspy --test detached_child
• cargo test -p vite_task_bin --test e2e_snapshots input_read_write_not_cached

[wyf027]

Closing this after seeing the maintainer request on #494 to stop submitting PRs unless an issue is assigned. Sorry for the noise.