Fix loop completion arcs on Python 3.14 - #2281

Fixes #2168.

On Python 3.14, both settrace-based cores can receive a return event after the frame's current line has advanced without a preceding line event for that final source line. They were therefore recording the function-exit arc from the previous line, which can make normal loop exhaustion immediately before return appear as a missing branch.

For real returns on Python 3.14+, ctrace and pytrace now record that final line transition first when needed, then record the function-exit arc. Sysmon already reports the expected loop-completion arc.

The regression covers empty, exhausted, and broken-loop paths. On Python 3.14.7 the exact reproducer reports 100% branch coverage with both ctrace and pytrace after the fix; the complete tests/test_arcs.py suite also passes under both cores. A broader return/loop/try-finally probe produced identical raw arc sets for ctrace and pytrace.

PyTracer's trace callback cannot itself be measured by meta-coverage, so the new callback branch uses the repository's pragma: not covered marker. A direct PyTracer return-frame test exercises the same transition outside the trace callback, while the end-to-end arc regression verifies the real tracing behavior.