Fix playback rate display dropping meaningful digits (#963)

PlaybackRate.displayText/compactDisplayText used String(format: "%.2gx"), whose significant-figure notation dropped meaningful digits (1.25 -> "1.2x") and rounded others (1.75 -> "1.8x"). Replace with a fixed two-decimal format + trailing-zero strip so whole rates have no decimal (2 -> "2x"), halves show one (1.5 -> "1.5x"), and quarter-steps are preserved (1.25 -> "1.25x"). Adds a Swift Testing suite (PlaybackRateTests) proving red->green.

YatteeTests/PlaybackRateTests.swift

@@ -0,0 +1,51 @@
+//
+//  PlaybackRateTests.swift
+//  YatteeTests
+//
+//  Tests for PlaybackRate display-text formatting.
+//
+
+import Testing
+import Foundation
+@testable import Yattee
+
+@Suite("PlaybackRate Display Tests")
+struct PlaybackRateDisplayTests {
+    @Test("displayText renders every rate without losing precision")
+    func displayTextPrecision() {
+        #expect(PlaybackRate.x025.displayText == "0.25x")
+        #expect(PlaybackRate.x05.displayText == "0.5x")
+        #expect(PlaybackRate.x075.displayText == "0.75x")
+        #expect(PlaybackRate.x125.displayText == "1.25x")
+        #expect(PlaybackRate.x15.displayText == "1.5x")
+        #expect(PlaybackRate.x175.displayText == "1.75x")
+        #expect(PlaybackRate.x2.displayText == "2x")
+        #expect(PlaybackRate.x25.displayText == "2.5x")
+        #expect(PlaybackRate.x3.displayText == "3x")
+    }
+
+    @Test("displayText for the normal rate uses the localized label, not the numeric form")
+    func displayTextNormal() {
+        let normal = PlaybackRate.x1.displayText
+        #expect(normal == String(localized: "player.playbackRate.normal"))
+        #expect(!normal.hasSuffix("x"))
+    }
+
+    @Test("compactDisplayText always shows the numeric value")
+    func compactDisplayText() {
+        #expect(PlaybackRate.x1.compactDisplayText == "1x")
+        #expect(PlaybackRate.x125.compactDisplayText == "1.25x")
+        #expect(PlaybackRate.x15.compactDisplayText == "1.5x")
+        #expect(PlaybackRate.x175.compactDisplayText == "1.75x")
+        #expect(PlaybackRate.x2.compactDisplayText == "2x")
+        #expect(PlaybackRate.x3.compactDisplayText == "3x")
+    }
+
+    @Test("every case has a non-empty display string")
+    func allCasesNonEmpty() {
+        for rate in PlaybackRate.allCases {
+            #expect(!rate.displayText.isEmpty)
+            #expect(!rate.compactDisplayText.isEmpty)
+        }
+    }
+}