Improve internal code organization as functionality grows

[darthfork]

The public and private split is good already, but more features will need more structure.

Tasks:

• Keep public API in zig/defs.bzl minimal.
• Move implementation details into zig/private/ as rules expand.
• Consider separating toolchain, providers, actions, and rule implementations into focused files.

Acceptance criteria:

• Internal layout stays understandable as new rules and providers are added.

Parent issue: #6