fix(engine): report diagnostic for empty source instead of IndexError

[SAY-5]

Compiling an empty source file crashed with IndexError: list index out of range at engine.py because compilable_line_obs[-1] was accessed on an empty list. The engine now reports a clear source file contains no compilable lines diagnostic in that case. Closes #53.

[michaelkamprath]

Thanks for the fix! The core change is correct — diagnostic_reporter.error() calls sys.exit(), so the IndexError crash on the next line is properly prevented. A few minor suggestions below.

[SAY-5]

Moved the guard before the sort and switched to not compilable_line_obs as suggested. On the comments-only variant: with the test config a comments-only file still produces line objects, so it does not exercise this empty-list path; kept the zero-byte baseline test instead.

[michaelkamprath]

Thanks for the contribution!

closes #53