test: add coverage for BoundedIDSet eviction

the gateway/bridge loop-prevention caches all sit on this type but it
had no direct unit tests. covers insert/contains, FIFO eviction at
capacity, re-insertion of an evicted id, and that a duplicate insert
doesn't disturb eviction order.

bitchatTests/Services/BoundedIDSetTests.swift

@@ -0,0 +1,87 @@
+//
+// BoundedIDSetTests.swift
+// bitchatTests
+//
+// Tests for BoundedIDSet insertion-ordered eviction
+//
+
+import Testing
+@testable import bitchat
+
+struct BoundedIDSetTests {
+
+    @Test func insertReturnsTrueForNewID() {
+        var set = BoundedIDSet(capacity: 3)
+        #expect(set.insert("a") == true)
+        #expect(set.contains("a"))
+    }
+
+    @Test func insertReturnsFalseForAlreadyPresentID() {
+        var set = BoundedIDSet(capacity: 3)
+        set.insert("a")
+        #expect(set.insert("a") == false)
+    }
+
+    @Test func containsIsFalseForUnknownID() {
+        let set = BoundedIDSet(capacity: 3)
+        #expect(set.contains("missing") == false)
+    }
+
+    @Test func evictsOldestWhenOverCapacity() {
+        var set = BoundedIDSet(capacity: 2)
+        set.insert("a")
+        set.insert("b")
+        set.insert("c")
+
+        #expect(set.contains("a") == false)
+        #expect(set.contains("b"))
+        #expect(set.contains("c"))
+    }
+
+    @Test func evictedIDCanBeReinserted() {
+        var set = BoundedIDSet(capacity: 1)
+        set.insert("a")
+        set.insert("b") // evicts "a"
+
+        #expect(set.insert("a") == true)
+        #expect(set.contains("a"))
+        #expect(set.contains("b") == false)
+    }
+
+    @Test func stayingUnderCapacityKeepsEverything() {
+        var set = BoundedIDSet(capacity: 5)
+        for id in ["a", "b", "c"] {
+            set.insert(id)
+        }
+
+        #expect(set.contains("a"))
+        #expect(set.contains("b"))
+        #expect(set.contains("c"))
+    }
+
+    @Test func capacityOneKeepsOnlyMostRecentID() {
+        var set = BoundedIDSet(capacity: 1)
+        set.insert("a")
+        set.insert("b")
+        set.insert("c")
+
+        #expect(set.contains("a") == false)
+        #expect(set.contains("b") == false)
+        #expect(set.contains("c"))
+    }
+
+    @Test func duplicateInsertDoesNotDisturbEvictionOrder() {
+        // Re-inserting an already-present ID is a no-op (returns false) and
+        // must not move it in the eviction queue or add a second entry.
+        var set = BoundedIDSet(capacity: 2)
+        set.insert("a")
+        set.insert("b")
+        set.insert("a") // no-op: "a" is already present
+
+        set.insert("c") // over capacity: should evict "a" (still the oldest)
+
+        #expect(set.contains("a") == false)
+        #expect(set.contains("b"))
+        #expect(set.contains("c"))
+    }
+}