Add deterministic process-signal lifecycle support to Repl.Testing

[autocarl]

Context

PR #80 adds process-wide first/second-signal coordination. Unit tests can exercise internal coordination seams, while real SIGINT/SIGTERM behavior currently requires spawning a child process and sending an operating-system signal. Consumers building on Repl.Testing need a supported, repeatable way to verify cleanup and exit-code contracts.

Desired outcome

Provide either:

1. a public in-memory test abstraction that can drive the logical process-signal lifecycle without real global OS registrations; or
2. first-class documented helpers for child-process signal tests when in-memory simulation would give false confidence.

A hybrid is likely appropriate: deterministic in-memory tests for policy/coordination and process tests for runtime/platform integration.

Acceptance criteria

• Tests can model first-signal cooperative cancellation and second-signal escalation deterministically.
• Tests can assert cleanup completion, callback failures, explicit exit-code precedence, and late joiners.
• The helper does not mutate real process-global signal state unless explicitly requested.
• Real-process guidance covers PID lifetime, output draining, timeouts, forced cleanup, and Unix/Windows differences.
• Repl.Testing documentation states which guarantees require a spawned process.