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>

Package.swift

@@ -53,11 +53,17 @@ let package = Package(
             path: "bitchatTests",
             exclude: [
                 "Info.plist",
-                "README.md"
+                "README.md",
+                // CI perf gate data (read by scripts/check-perf-floors.sh),
+                // not a test resource.
+                "Performance/perf-floors.json"
             ],
             resources: [
                 .process("Localization"),
-                .process("Noise")
+                // Only the vector fixture: declaring the whole "Noise"
+                // directory would claim its .swift test files as resources
+                // and silently drop them from compilation.
+                .process("Noise/NoiseTestVectors.json")
             ]
         )
     ]