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.lock

@@ -1009,6 +1009,25 @@ version = "1.16.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ba467056f1b547ed52077911161fc86985becbc60e8e1857c8a144dab0def891"
 
+[[package]]
+name = "sn-exit"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "thiserror",
+ "tokio",
+ "tracing",
+ "tracing-subscriber",
+]
+
+[[package]]
+name = "sn-fair"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "thiserror",
+]
+
 [[package]]
 name = "sn-fetch"
 version = "0.1.0"
@@ -1024,6 +1043,16 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "sn-node"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "serde_json",
+ "thiserror",
+ "tokio",
+]
+
 [[package]]
 name = "socket2"
 version = "0.6.5"