Add staggered parallel worker startup

[espekkaya]

🤔 What's changed?

Adds native staggered startup for parallel Cucumber workers.

New configuration options:

• --parallel-startup-mode=staggered
• --parallel-ramp-size=5
• --parallel-ramp-delay=1000

In staggered mode, Cucumber starts an initial group of workers, begins executing scenarios as soon as a worker has completed its worker-level BeforeAll hooks, and progressively starts further workers as capacity becomes available.

The default eager startup mode is unchanged.

This includes:

• CLI parsing, configuration schema, public types, and API reports
• Worker lifecycle changes in the parallel runtime
• Documentation and changelog updates
• Acceptance coverage that verifies scenarios can begin before later workers complete their worker-level hooks

⚡️ What's your motivation?

Large --parallel values can create a significant worker startup spike before the first scenario begins. This is especially noticeable in CI environments.

Staggered startup reduces time-to-first-scenario by allowing early workers to begin execution while the remaining worker pool is still being prepared.

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)
• 📖 Documentation

♻️ Anything particular you want feedback on?

Feedback on the ramp semantics would be especially useful:

• parallelRampSize controls the number of workers created initially.
• parallelRampDelay controls the minimum delay between workers created after the initial group.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG.