Define xce_dispatch_fail_site_t/xce_dispatch_fail_class_t and
xce_classify_dispatch_fail() in job.c, mapping each of dispatch_a_job_()'s
six post-lookup failure points to retryable or terminal per Requirement
4.1/4.2. Standalone, not yet wired into dispatch_a_job_() (deferred to
tasks 3.2/3.3). Also adds this spec's requirements/design/research/tasks
docs (first commit touching this spec).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
