Fix signed varint overflow on Linux

Use wrapping subtraction (0 &- x) instead of (~x + 1) to avoid
arithmetic overflow with Swift's strict integer checking.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: JustinMeans

Sources/Transmission/BinarySerializer.swift

@@ -106,7 +106,7 @@ public struct CompactDecoder: Sendable {
 
     public mutating func readVarintSigned() throws -> Int64 {
         let unsigned = try readVarint()
-        let signed = Int64(bitPattern: (unsigned >> 1) ^ (~(unsigned & 1) + 1))
+        let signed = Int64(bitPattern: (unsigned >> 1) ^ (0 &- (unsigned & 1)))
         return signed
     }