test: Cover ^C escalation and task abandonment

Ladder scenarios on top of the core ^C delivery tests: the rescue timer's
escalation offers and rung climbing (subprocess and pty, at several thread
counts), direct abandonment of unresponsive compute, the
catch-all-swallowing loop that cannot hide from ^C, REPL backend rescue
after an abandonment, and the threaded cancellation exec suite.

This pull request was written with the assistance of generative AI.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Keno