New alloc_runner.c: sched_allocator_run() validates its inputs (-EINVAL
without touching the pend reason), applies the plugin's optional accepts
guard, creates the pend-reason record, then checks shared resources: one
host-less NO_RESOURCE reason per unmet entry (skip only on vget < 0,
fixing bug-1893) and the met amounts counted once as uncommitted. Plan
search follows in task 3.2. Runner tests use a fake allocator contract;
the pend-reason factory audit now scans the runner.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>