Add retry allowance controls

[puffball1567]

Summary

• add keyed RetryAllowance controls for bounding retry storms by dependency, tenant, workflow, or worker class
• add retry allowance metric conversion, test coverage, and API/support documentation
• document independent naming/API choices and public-pattern diligence in IP notes

Verification

• nim r --nimcache:/tmp/flowbrigade-retry-allowance-all-nimcache -p:src tests/all.nim
• nim r --nimcache:/tmp/flowbrigade-retry-allowance-metrics-nimcache -p:src tests/test_metrics.nim
• nimble check