OSDiagnosticsLogger was using print() for actual log output despite being
the 'OSLog-based logger' implementation. Per AGENTS.md and the existing
struct name, it should use os.Logger.
Changes:
+ import os
+ private let logger: Logger (initialised in init())
+ logger.debug / info / warning / error replace print() at all 4 sites
+ uses privacy: .public to make messages visible in Console.app
scripts/check-rule-violations.sh shows 4 \u2192 0 swift-print findings in
this package. The common-plat repo now contributes 0 swift-print to
the ecosystem total.
