Implemented phase 1 for the POC prototype in the Website service.

Services/Website/Tests/Library/Cases/Internal/Extensions/ListingDisplayTests.swift

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+import Persistence
+import Testing
+
+@testable import WebsiteLibrary
+
+@Suite(
+    "Listing+Display extension",
+    .tags(.extensionTests)
+)
+struct ListingDisplayTests {
+
+    // MARK: Computed tests
+
+    @Test(arguments: zip(
+        [9.8, 9.0, 8.5, 7.0, 6.5],
+        [Chip.Tone.good, .good, .neutral, .neutral, .warn]
+    ))
+    func `tones a slab's chip by its grade`(
+        grade: Double,
+        expects tone: Chip.Tone
+    ) {
+        let chip = Listing.stub(grade: grade).chip
+
+        #expect(chip.label == "CGC \(grade)")
+        #expect(chip.tone == tone)
+        #expect(chip.isCertified)
+    }
+
+    @Test
+    func `marks a raw issue's chip as seller-graded`() {
+        let chip = Listing.stub(grader: nil, grade: 9.4, condition: "NM").chip
+
+        #expect(chip.label == "Raw · NM")
+        #expect(chip.tone == .neutral)
+        #expect(!chip.isCertified)
+    }
+
+    @Test
+    func `states a collected edition's condition alone`() {
+        #expect(Listing.stub(format: "manga", grader: nil, grade: nil, condition: "Like new").chip.label == "Like new")
+    }
+
+    @Test
+    func `leads a collected edition's summary with its format`() {
+        #expect(Listing.stub().summary == "Meridian Comics · 2019 · 1st print")
+        #expect(Listing.stub(format: "hardcovers").summary == "Hardcover · Meridian Comics · 2019 · 1st print")
+    }
+
+    @Test
+    func `derives the paths from the slugs`() {
+        #expect(Listing.stub().path == "/listing/nightward-1")
+        #expect(Shop.stub().path == "/shop/kestrel-row-comics")
+    }
+
+    @Test(arguments: zip(
+        ["Kestrel Row Comics", "Inkwell & Staple", "Silverfish"],
+        ["KR", "IS", "S"]
+    ))
+    func `monogram`(
+        of name: String,
+        expects monogram: String
+    ) {
+        #expect(Shop.stub(name: name).monogram == monogram)
+    }
+
+    @Test(arguments: zip(
+        [24_000, 1_250, 5, 0],
+        ["€240", "€12.50", "€0.05", "€0"]
+    ))
+    func `euros`(
+        of cents: Int,
+        expects price: String
+    ) {
+        #expect(cents.euros == price)
+    }
+
+    @Test
+    func `returns to the gated action's path after sign-in`() {
+        #expect(String.Route.signIn(returningTo: "/shop/a?keys=1&page=2") == "/sign-in?return=/shop/a%3Fkeys%3D1%26page%3D2")
+    }
+
+}