feat: export translated C headers as a public Zig module

[wemeetagain]

Summary

• translate the upstream blst.h declarations, including blst_aux.h, at build time
• expose the translated bindings as the public blst Zig module
• link the native blst library transitively for module consumers
• document module-based usage and add integration-test coverage in CI

Why

Consumers currently receive only the compiled library artifact and must arrange C declarations and linking themselves. Exporting a public module allows applications to use @import("blst") while retaining the target-specific bindings and native implementation supplied by this package.

[spiral-ladder]

lgtm