Fix registry keyed limiter clear

[puffball1567]

Summary

• wire registry-created keyed fixed-window limiters to their underlying clear operation
• add registry and C ABI regression coverage for clearing named keyed limiters

Context

This was found while exercising the Rust C ABI bindings: direct keyed fixed-window clear worked, stored limiter clear worked, but registry-created keyed fixed-window clear always returned false.

Verification

• nim r --nimcache:/tmp/flowbrigade-registry-clear-registry-nimcache -p:src tests/test_ratelimit_registry.nim
• nim r --nimcache:/tmp/flowbrigade-registry-clear-cabi-nimcache -p:src tests/test_c_abi.nim
• nim r --nimcache:/tmp/flowbrigade-registry-clear-all-nimcache -p:src tests/all.nim
• nimble check