feat: add fair-share allocation between queues

[sylvesterdamgaard]

Summary

Adds a fairness layer between per-queue demand evaluation and cross-host distribution, resolving the first-queue-wins starvation problem when total demand exceeds cluster capacity.

• New FairShareAllocator class: min-first then proportional allocation with water-filling iteration to reclaim capacity freed by max clamping
• Refactored evaluateAndPublishClusterRecommendations() from single evaluate-and-distribute loop into three phases: collect demands → fair-share allocate → distribute adjusted targets
• Pinned (non-scalable) workloads bypass the allocator and subtract from available capacity
• No new configuration keys — workers.max remains a safety bound, not a fairness mechanism
• 16 new tests (14 unit + 2 integration), all 496 tests passing

Key behaviors

• No contention → no change: if sum(demand) <= capacity, every queue gets exactly what it asked for
• Mins guaranteed: every queue gets at least workers.min, even under pressure
• Idle queues yield: capacity flows to queues that need it
• Max as safety bound: proportional distribution + water-filling ensures max clamping doesn't waste capacity
• Deterministic: tie-breaking by workload key ensures stable, reproducible allocations

Closes #16

Test plan

• No contention returns demands unchanged
• Equal contention distributes proportionally with largest-remainder
• Min guarantees hold even when mins exceed capacity
• Idle queues yield all capacity to busy queues
• Max clamping redistributes freed capacity via water-filling
• Multiple queues hitting max in sequence converge correctly
• Deterministic results regardless of input order
• Fair-share + distributeClusterTarget integration (multi-host)
• Full test suite passes (496 tests, 1241 assertions)
• PHPStan clean (1 pre-existing unrelated error)
• Pint clean

🤖 Generated with Claude Code