Add rate limiter features

[puffball1567]

Summary

• add in-process GCRA-style and keyed GCRA-style rate limiters
• cover inspection, consume, reset, key capacity, unsafe key rejection, and fractional interval behavior
• add benchmark coverage and documentation updates, including IP due-diligence notes for standards-described algorithms

Verification

• nim r --nimcache:/tmp/flowbrigade-main-gcra-nimcache -p:src tests/test_ratelimit_gcra.nim
• nim r --nimcache:/tmp/flowbrigade-main-all-after-gcra-nimcache -p:src tests/all.nim
• nimble benchmark
• nimble check