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/Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["crates/sn-fetch", "crates/sn-node", "crates/sn-exit"]
+members = ["crates/sn-fetch", "crates/sn-node", "crates/sn-exit", "crates/sn-fair"]
 
 [workspace.package]
 version = "0.1.0"