v1.2.0 of nemu_tracking_flutter

Nemu Smart Links attribution SDK for Flutter - UTM tracking, deep linking, and install attribution. Changelog excerpt: - Add support for Nemu-format UTM parameters (nemu_source, nemu_medium, nemu_adset, nemu_campaign, nemu_content, nemu_term). - When any nemu_*parameter is present, the Nemu set takes full precedence and traditional utm_*parameters are ignored (no merging). utm_mediumresolves from nemu_adsetfirst, then nemu_medium. - URLs with only traditional utm_*parameters keep the existing behavior. - Nemu keys are excluded from custom_params.