Decide and document interop strategy with C/C++ and system libraries

[darthfork]

Zig often interfaces with C toolchains and system libraries. Users will eventually need this.

Tasks:

• Decide whether first release supports C interop or explicitly defers it.
• Document the support boundary.
• If supported, design provider and toolchain interop with Bazel C++ rules.

Acceptance criteria:

• README or docs clearly state what interop is and is not supported.
• If supported, examples and tests exist.

Parent issue: #3