Observe notifications are NON-only, no CON support

[cvik]

Severity: MEDIUM (from external code review)

Server observe always sends NON notifications. RFC 7641 expects periodic CON notifications for reliability and no outstanding-notification tracking per client.

Fix: Add option for CON notifications with retransmit tracking.

[cvik]

Fixed in PR #92 — CON notifications with configurable interval, retransmit, and observer eviction on timeout.