[BACKEND] Add iluvatar compute spec key specialization

[stringl1l1l1l]

Summary

This PR moves iluvatar compute_spec_key specialization behind the FlagTree backend spec mechanism, keeping the default Triton JIT path unchanged while enabling iluvatar-specific 4-byte divisibility keying.

What’s included

• add a spec_func("compute_spec_key") lookup in JIT binder creation
• add iluvatar compute_spec_key implementation using 4-byte divisibility
• export the new spec function from third_party/iluvatar/backend/spec/__init__.py

Notes

• This follows the existing FlagTree backend specialization pattern: mainline code exposes only a generic hook, while iluvatar-specific behavior lives under third_party/iluvatar/backend/spec/....

[CLAassistant]

All committers have signed the CLA.