refactor(traffic): replace package-vars with Scheduler interface and struct

[maansaake]

Summary

Refactors pkg/traffic to eliminate all package-level mutable state, replacing it with a proper struct-based design backed by an exported Scheduler interface.

Changes

pkg/traffic/scheduler.go

• Add Opts struct with Logger, WorkerLimit, SampleTolerancePerc fields
• Add Scheduler interface with Run(ctx, metadata, reporter) error and Stop() error methods
• Add unexported scheduler struct implementing Scheduler
• Add New(opts *Opts) Scheduler constructor with defaulting
• Remove all package-level variables: reporter, workloads, stopChan, logger, SampleTolerancePerc
• Remove package-level Run() and Stop() functions

pkg/traffic/workload.go

• Add reporter, stopChan, logger as injected struct fields
• Replace all references to package-level reporter, stopChan, and logger

pkg/traffic/worker.go

• Replace logger.X calls with worker.parent.logger.X

pkg/traffic/scheduler_test.go

• Add newTestScheduler() helper using New(&Opts{...})
• Update all tests to call .Run() / .Stop() on the scheduler instance

arbiter.go

• Create sched := traffic.New(&traffic.Opts{Logger: logger, WorkerLimit: ...}) at the start of run()
• Replace traffic.Run() / traffic.Stop() calls with sched.Run() / sched.Stop()

Validation

make validate passes: lint clean, all unit tests green, sample example runs successfully.