fix(examples/zig): upgrade to zig 0.16 to unbreak the macOS test

[mikeland73]

Summary

The zig-hello-world example test has been failing on macOS, which blocks every release since cli-release gates on the test suite. devbox.lock pinned zig 0.11.0 (from a Feb 2024 nixpkgs commit), whose self-hosted Mach-O linker emits __DATA_CONST segments without the SG_READ_ONLY flag — modern macOS dyld rejects these, and the binary that crashed was zig's own build runner, hence the opaque the following build command crashed (Linux was unaffected). This pins zig explicitly at 0.16.0 rather than tracking latest, since zig's build API breaks across nearly every release and an unpinned update would silently re-break build.zig — matching how go and nim are already pinned under examples/. Upgrading exposed a second latent problem, so build.zig is migrated from the pre-0.12 .root_source_file = .{ .path = ... } form to the 0.15+ root_module API for both the executable and the test. Finally, .gitignore is corrected to .zig-cache/, since zig 0.12 renamed the cache directory and build output was no longer being ignored.

How was it tested?

Reproduced the original dyld: __DATA_CONST segment missing SG_READ_ONLY flag crash locally on macos-arm64 (same architecture as the macos-latest runner), then confirmed the fix through the exact CI harness rather than a bare zig build:

DEVBOX_RUN_PROJECT_TESTS=1 go test ./testscripts -run 'TestExamples/development_zig_zig-hello-world_run_test' -v

Build Summary: 6/6 steps succeeded; 1/1 tests passed
--- PASS: TestExamples/development_zig_zig-hello-world_run_test.test (2.56s)
ok go.jetify.com/devbox/testscripts

The Linux shard was not run locally, but both changes are platform-independent (build API) and already green there. All four systems resolve in the updated lock.

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