Clarify target-platform vs exec-platform behavior

[darthfork]

The ruleset has the beginnings of toolchain integration, but production language rulesets need a clear model for host tools, execution platform, and target platform, especially for cross-compilation.

Current code:

• zig/extensions.bzl
• zig/toolchain.bzl
• zig/rules.bzl

Tasks:

• Document the intended meaning of the Zig toolchain in Bazel terms.
• Verify toolchain selection works correctly for exec and target platforms.
• Design for cross-compilation via --platforms.
• Add tests covering at least one cross-platform build scenario.

Acceptance criteria:

• Cross-platform behavior is documented.
• Toolchain selection is validated by tests.
• Users can intentionally cross-compile with documented examples.

Parent issue: #2

[darthfork]

Leaving this open, but it should probably be re-scoped. PR #22 documented the current exec-platform behavior, but true target-platform-driven cross-compilation via --platforms is still not implemented. Suggested next step: narrow this issue to cross-compilation design + tests instead of general documentation.