Add settings validation with Warning and Error severity levels.
Warnings print to stderr without preventing startup; errors are
aggregated and prevent startup. Percentage validation accepts 1..=100
rather than silently clamping. Add UCI-compatible flexible boolean
parsing through Serde, accepting 1/true/yes/y/on/enabled for true and
0/false/no/n/off/disabled for false, while native booleans continue to
deserialize correctly.
