Expand C ABI rate limiter surface

[puffball1567]

Summary

• expose GCRA, keyed GCRA, and retry allowance through the experimental C ABI
• bump the core package version to 0.4.0 and C ABI version to 2
• update C ABI docs, README coverage, roadmap, and test matrix

Verification

• nim r --nimcache:/tmp/flowbrigade-cabi-gcra-retry-all-nimcache -p:src tests/all.nim
• nimble check
• nimble cabi
• gcc -Iinclude tests/c_abi_smoke.c -L/tmp -lflowbrigade -Wl,-rpath,/tmp -o /tmp/flowbrigade-c-abi-smoke
• /tmp/flowbrigade-c-abi-smoke
• gcc -Iinclude examples/c_abi_quickstart.c -L/tmp -lflowbrigade -Wl,-rpath,/tmp -o /tmp/flowbrigade-c-abi-example
• /tmp/flowbrigade-c-abi-example