test: cover chat export formatting and sealing warning copy

Verify scope titles, chronological ordering, and that empty exports still
surface the unsealed-plaintext warning line.

bitchatTests/ChatPlaintextExporterTests.swift

@@ -0,0 +1,54 @@
+import BitFoundation
+import Foundation
+import Testing
+
+struct ChatPlaintextExporterTests {
+    private static let fixedDate = Date(timeIntervalSince1970: 1_700_000_000)
+
+    @Test func exportIncludesSealingWarningAndScope() {
+        let message = BitchatMessage(
+            sender: "alice",
+            content: "hello mesh",
+            timestamp: Self.fixedDate,
+            isRelay: false
+        )
+        let text = ChatPlaintextExporter.exportText(
+            messages: [message],
+            context: ChatPlaintextExporter.Context(scopeTitle: "mesh", exportedAt: Self.fixedDate)
+        )
+        #expect(text.contains("bitchat export — mesh"))
+        #expect(text.contains("warning: this file is unsealed plaintext"))
+        #expect(text.contains("alice: hello mesh"))
+    }
+
+    @Test func emptyConversationStillWarns() {
+        let text = ChatPlaintextExporter.exportText(
+            messages: [],
+            context: ChatPlaintextExporter.Context(scopeTitle: "dm with @bob", exportedAt: Self.fixedDate)
+        )
+        #expect(text.contains("warning: this file is unsealed plaintext"))
+        #expect(text.contains("(no messages in this conversation)"))
+    }
+
+    @Test func messagesPreserveChronologicalOrder() {
+        let first = BitchatMessage(
+            sender: "a",
+            content: "one",
+            timestamp: Self.fixedDate,
+            isRelay: false
+        )
+        let second = BitchatMessage(
+            sender: "b",
+            content: "two",
+            timestamp: Self.fixedDate.addingTimeInterval(60),
+            isRelay: false
+        )
+        let text = ChatPlaintextExporter.exportText(
+            messages: [first, second],
+            context: ChatPlaintextExporter.Context(scopeTitle: "mesh", exportedAt: Self.fixedDate)
+        )
+        let oneRange = text.range(of: "one")!
+        let twoRange = text.range(of: "two")!
+        #expect(oneRange.lowerBound < twoRange.lowerBound)
+    }
+}