Add std JobObject kill-on-close opt-in

[kreneskyp]

Refs IR-783.

Add a safe std KillOnDrop marker that opts a Windows JobObject into kill-on-close. Keep the existing std behavior unchanged when the marker is absent; the existing suspended spawn, job assignment, and resume sequence remains in place. The option works in either wrapper registration order.

The Windows integration tests check default behavior, opt-in termination of a child and descendant when the wrapper is dropped, and that failed job assignment terminates the suspended child before its marker command runs.

Local macOS checks:

• cargo fmt --check passed.
• cargo test --locked --features std --all-targets passed (55 tests).
• cargo test --locked --no-default-features --features std --all-targets passed (31 tests).
• cargo test --locked --all-features --all-targets passed.
• cargo clippy --locked --features std passed with an existing unused-import warning in src/std/core.rs.
• cargo check --locked --no-default-features --lib passed.
• The workflow multiprocess helper passed with one child.
• Windows std_windows test target cross-compiles with rustup stable on macOS; this is compile evidence, not runtime execution.

Native Windows execution and CI are pending.

[passcod]

From other PRs by account this appears to be fully-automated AI submissions. Come back with a human touch.