resource reservations

[nick-potts]

Implemented adaptive scheduling and surfaced reservations in the CLI:

• Added a no-reservation ranker to keep round-robin spreading when CPU/memory reservations are zero, while retaining the existing spread ranker (existing+scheduled) when reservations are set. Tie-breaking stays deterministic.
• Updated scheduler/strategy tests to cover adaptive ranking, reservation rollback, port conflicts, force-recreate, volume constraints, and resource accounting.
• Added scheduling docs for clarity, documenting eligibility, adaptive ranking, resource accounting, modes, conflicts, and troubleshooting; updated the global vs replicated table to match current behavior.
• Exposed --reserve-cpu and --reserve-memory on uc service run to set CPUReservation/MemoryReservation, aligning ad-hoc runs with deploy/Compose scheduling.

Design choices:

• Opted for an implicit adaptive ranker instead of a new x-spread knob; balanced behavior remains for reserved workloads, round-robin for best-effort workloads.
• CLI change is additive and backward compatible; limits remain as before, reservations now available for scheduling-aware runs.