Resurrect dead Noise vector tests; add CI perf floors; make tests hermetic

Package.swift's .process("Noise") resource claim silently excluded all
of bitchatTests/Noise/ from compilation since Oct 2025 - including a
complete official-vector runner (cacophony + snow XX transcripts,
transport messages, handshake hash, byte-identical to upstream).
Narrowing the resource to the JSON file and loading via Bundle.module
brings 51 Noise tests back to life, with a guard asserting each
vector's protocol name matches the app's.

CI gains a performance floor gate: perf-floors.json carries deliberately
generous floors (~25% of measured throughput) that catch algorithmic
regressions without flaking on runner variance; PERF lines reach the
gate via an O_APPEND side-channel file since swift test --parallel
swallows passing tests' stdout.

Tests are now hermetic: FavoritesPersistenceService uses an in-memory
keychain under test (fixes the securityd hang that blocked pipeline
benchmarks locally) and read-receipt persistence uses a wiped scratch
UserDefaults suite instead of .standard.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchat/Services/TestEnvironment.swift

@@ -0,0 +1,25 @@
+//
+// TestEnvironment.swift
+// bitchat
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Foundation
+
+/// Process-level test-environment detection for singletons that must swap a
+/// real OS-backed dependency (keychain, persistent defaults, notifications)
+/// for an in-memory one under test. Mirrors the detection already used by
+/// `NotificationService` and `LocationStateManager`.
+enum TestEnvironment {
+    /// True when running under XCTest / Swift Testing or in CI.
+    static let isRunningTests: Bool = {
+        let env = ProcessInfo.processInfo.environment
+        return NSClassFromString("XCTestCase") != nil ||
+               env["XCTestConfigurationFilePath"] != nil ||
+               env["XCTestBundlePath"] != nil ||
+               env["GITHUB_ACTIONS"] != nil ||
+               env["CI"] != nil
+    }()
+}