Hot Restart Launched Target App

[drown0315]

Parent

#98

What to build

Expose hot restart as an operation on the launched Target App process. The
Target App Launcher should own the Flutter process communication needed to
request a hot restart and report success or failure through a small testable
interface. This slice should not orchestrate a full Project Run; it only makes
the launched app reusable between Scenarios.

Acceptance criteria

• A launched Target App handle exposes a hot restart operation.
• Hot restart uses Flutter process control rather than Scenario semantics.
• Hot restart success is observable by callers.
• Hot restart failure returns a structured launch/process-level failure.
• Existing cleanup behavior remains intact.
• Fake process tests cover hot restart success.
• Fake process tests cover hot restart failure.
• Fake process tests cover cleanup after hot restart activity.

Blocked by

None - can start immediately.