Closes #3.
Replaces the placeholder-only test files with behavioral coverage, wires the hooks/mock/snapshot/time integration executables into zig build test, and makes zig build examples compile-check every maintained example while still running the fast core examples.
The compile-only examples intentionally demonstrate delays, generated files, interactive output, or failure handling; this distinction is now documented. While enabling them, this also updates stale Zig 0.17 compatibility calls and fixes MultiSpinner's anonymous struct type mismatch.
This PR is stacked on #17 so its diff stays scoped to issue #3. Merge #17 first, then retarget this PR to main.
Verification:
zig fmt --check .zig buildzig build testzig build exampleszig build -Dtarget=x86_64-linux-gnu- clean worktree after generated snapshot/coverage artifacts