Add sn-fair v0: weighted max-min fairness scheduler with leech policy

Two-class allocation engine: contributors (configurable weight) get
weighted max-min shares of the pooled capacity, leeches (receive-only,
explicitly allowed) share the leftover capacity. Deterministic u128
arithmetic, validated newtypes, demo CLI in Mbit/s for the pilot
Soll/Ist comparison. 19 tests, workspace clippy clean.

server/crates/sn-fair/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "sn-fair"
+description = "Weighted max-min fairness scheduler for SpiderNet neighborhood networks"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+thiserror = { workspace = true }
+clap = { workspace = true }
+
+[lints]
+workspace = true