fix: Campus.run() double-stop with context manager #20

What to build

When using with Campus(...) as c: c.run(), start() is called in __enter__, then run() calls start() again (double start), and stop() is called in finally then again in __exit__.

Add an idempotency guard: if self._running: return at the top of start(), or remove the start() call from run().

Acceptance criteria

• Context manager + run() lifecycle is idempotent
• Tests verify single start/stop cycle

Blocked by

None - can start immediately

Parent

Tracks the campus framework v1 code review: #27