Move the "which failures can happen before Starting" classification onto
FailureReason as an exhaustive predicate and use it for every
Runnable -> Failed transition, replacing the hand-maintained allowlist.
Give the pending-cancel precedence rule a single owner shared by the
worker launch and worker exit paths.
Test cleanups: share the Notify wait loop, server record fixture, and
post-failure assertions; simplify the pre-start test runtime's hold
flag; and parameterize the slate run.failed payload helper.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>