fix: sort RC builds by rcNumber within same version (#210)

RCs were all given rank (1, 0) regardless of rcNumber, causing RC 2
to sort incorrectly relative to RC when falling back to build number
string comparison.

Signed-off-by: Patrick Linnane <patrick@linnane.io>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: p-linnane

Sources/macOSdbKit/Models/Release.swift

@@ -214,7 +214,7 @@ extension Release: Comparable {
 
     private var prereleaseRank: (Int, Int) {
         if isBeta { return (0, betaNumber ?? 0) }
-        if isRC { return (1, 0) }
+        if isRC { return (1, rcNumber ?? 0) }
         return (2, 0)
     }
 }