feat: add weighted fair queuing scheduler

Introduces a scheduler with four layered admission constraints:

- Total concurrency: hard global cap on running jobs
- Priority tiers: weighted share of total concurrency per tier
  (Priority struct with Level + Weight)
- Per-type limits: fraction of tier slots per job type
- Conflict groups: mutual exclusion by group + job ID

Within a tier, jobs are ordered by accumulated cost per fairness
key (lowest first), with cost estimated via EMA of wall time.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alecthomas