Add configurable request queues and load shedding

[samuel-williams-shopify]

Summary

• add an immutable Ruby configuration DSL for multiple request queues
• centralize worker permit matching in a scheduler with FIFO arbitration across queue heads
• add spread and affinity-packing balance policies, hard worker eligibility, and extensible policy objects
• add per-queue depth/wait limits, a global pending limit, and configurable 429/Retry-After shedding
• preserve the existing single-queue, single-permit behavior by default

Scheduling model

Requests follow:

classify -> admit/reject -> enqueue -> match permit -> dispatch

Only the central scheduler consumes permits. Queue affinity remains soft and work-conserving: when the preferred worker has no permit, another eligible worker can accept the request.

Configuration

Applications can explicitly load trusted configuration from config/fantail.rb using Fantail::Configuration.load. Configuration is finalized and frozen before use.

Testing

• bundle exec bake test
• bundle exec rubocop
• 35 examples, 74 assertions
• 100% line coverage
• 99/99 public definitions documented