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/src/error.rs

@@ -0,0 +1,24 @@
+//! Error type for the fairness scheduler.
+
+use crate::types::ParticipantId;
+
+#[derive(Debug, thiserror::Error)]
+pub enum FairnessError {
+    #[error("capacity must be greater than zero")]
+    ZeroCapacity,
+
+    #[error("invalid participant id `{0}`: expected a decimal number greater than zero")]
+    InvalidParticipantId(String),
+
+    #[error("demand references unknown participant {0}")]
+    UnknownParticipant(ParticipantId),
+
+    #[error("duplicate demand for participant {0}")]
+    DuplicateDemand(ParticipantId),
+
+    #[error("duplicate participant {0}")]
+    DuplicateParticipant(ParticipantId),
+
+    #[error("conflicting participation classes for participant {0}")]
+    ConflictingParticipation(ParticipantId),
+}