These three packages had no tests. Add behavioural, external
(package foo_test) tests that can each fail on a plausible break:
- globals: setters and read-back through New, plus that New returns
an independent snapshot. One sequential function with a disclosed
paralleltest suppression, because it mutates shared package globals.
- healthcheck: Check returns status "ok", the exact documented JSON
field set, an RFC3339Nano timestamp, maintenanceMode mirroring the
config in both states, and the version and appname from globals.
- logger: New yields a usable *slog.Logger, debug is suppressed at the
default level, and EnableDebugLogging turns it on.
No production code changed. TTY-branch output format is not asserted.
Model: opus-4-8
