Add severity(forCrap:threshold:) band classifier

Sources/CrapCore/Severity.swift

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+import Foundation
+
+/// Human-readable severity band for a CRAP score, relative to a threshold.
+/// (Deliberately shipped without tests to demonstrate crap4swift flagging it.)
+public func severity(forCrap crap: Double, threshold: Double) -> String {
+    if crap <= threshold * 0.25 { return "trivial" }
+    if crap <= threshold * 0.5 { return "low" }
+    if crap <= threshold { return "watch" }
+    if crap <= threshold * 3 { return "high" }
+    if crap <= threshold * 10 { return "severe" }
+    return "critical"
+}

Tests/CrapCoreTests/SeverityTests.swift

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+import Testing
+@testable import CrapCore
+
+@Suite("CRAP severity bands")
+struct SeverityTests {
+    @Test("classifies each band relative to the threshold", arguments: [
+        (10.0, "trivial"),   // <= 25
+        (40.0, "low"),       // <= 50
+        (80.0, "watch"),     // <= 100
+        (200.0, "high"),     // <= 300
+        (800.0, "severe"),   // <= 1000
+        (2000.0, "critical"),
+    ])
+    func bands(crap: Double, expected: String) {
+        #expect(severity(forCrap: crap, threshold: 100) == expected)
+    }
+}