feat: library update after SDS-R

[darshankabariya]

why

• SDS-R gates every repair code path on a non-empty participantId, but the C constructor has no way to set one — so through the FFI, repair is silently a no-op.
• ReliabilityConfig is also hardcoded to defaults, leaving consumers unable to tune bloom-filter sizing, repair backoff, or response-group count for production.
• There's also no way to remove a channel, so per-channel state grows over time.

What

• New constructor that takes a participantId and a JSON-encoded config.
• New SdsRemoveChannel entry point.
• Existing constructor kept as a back-compat shim.

JSON is used for the config because it keeps the ABI forward-compatible