test(run): synchronize PTY observation fixtures

[schickling-assistant]

Summary

• expose test-only child-spawn observation hooks for PTY metadata and inventory commands
• synchronize fake PTY scripts with the parent test before mutating or reading their fixtures
• remove timing races from the Darwin PTY observation tests without changing production behavior

Why

The downstream dotfiles package carried this patch because several PTY observation tests raced the fake child scripts on Darwin. The tests now use an explicit ready/release barrier instead of assuming process scheduling order.

Verification

• the five patched st2::run tests pass on aarch64-darwin in nix develop
• nix build .#st2 --no-link -L in dotfiles passed with this patch and ran the full st2 library suite (691 tests)
• the upstream full workspace test run reached 684 passing tests; eight unrelated existing concurrency/path tests failed, while a representative failure passed alone
• git diff --check

The repository-wide cargo fmt --all --check is red on unchanged files under the current flake toolchain, so this PR does not reformat unrelated code.

Posted on behalf of @schickling

field	value
agent_identity	mbp2025.direct.omp.5tek2r48
session	mbp2025.5tek2r48
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.0.11
agent_runtime	OMP 18.0.11
tooling_profile	dotfiles@4cc7b25