Migrate to new FutureSDR/slint APIs and rasterize plot SVG ourselves

[JuliDi]

• Adopt FutureSDR's #[derive(Block)] kernel model: AdditiveNoise and ChannelSink use #[input]/#[output] fields, message handlers declared via #[message_inputs], and the flowgraph driver moves to the new prelude + RunningFlowgraph::call/stop_and_wait. The old per-edge Slab::with_config syntax is gone; default buffers replace it.
• slint 1.16 pulls in fontique 0.8 transitively, but plotters' global yeslogic-fontconfig-sys/dlopen breaks fontique's non-dlopen symbol imports. Re-route through i-slint-common's fontconfig-dlopen feature so the unification matches.
• slint's Image::load_from_svg_data parses with usvg::Options::default() (empty fontdb) and silently drops , so axis labels disappeared. Parse the plotters SVG ourselves with resvg/usvg using a fontdb populated from system fonts, render via tiny_skia, and hand slint a finished bitmap.
• Add justfile and reference its recipes from the README.