Q-W1 pt1: extract core/json (json::Reader/Writer), collapse wire Cursor family into core/wire, shared readFileBytes — five JSON decoders and three cursor copies deleted, byte-identical formats, 59/59 green

tests/test_tail_control.cpp

@@ -135,6 +135,20 @@ static void testRoundTripAuto() {
     CHECK(back && settingsEqual(*back, s));
 }
 
+static void testSerializeByteIdentity() {
+    // Q-W1 structural-dedupe guard: the core/json-backed writer must emit the
+    // EXACT bytes the former snprintf writer produced ({"mode":%d,"manualMs":%.17g})
+    // and re-serializing a round-tripped setting must be byte-identical — the blob
+    // lives in the .rpp, so a byte shift would dirty every saved project.
+    TailSetting s;  // None + 2000.0 default
+    CHECK(serializeTailSetting(s) == "{\"mode\":0,\"manualMs\":2000}");
+    TailSetting man; man.mode = TailMode::Manual; man.manualMs = 3141.592653589793;
+    const std::string json = serializeTailSetting(man);
+    auto back = deserializeTailSetting(json);
+    CHECK(back.has_value());
+    CHECK(back && serializeTailSetting(*back) == json);  // stable second round-trip
+}
+
 static void testDeserializeEmptyIsDefault() {
     // An absent/empty stored value (older project) -> nullopt, so the caller falls
     // back to the default. This is the graceful-old-project path the brief requires.
@@ -164,6 +178,7 @@ int main() {
     testRoundTripNoneDefault();
     testRoundTripManualArbitraryMs();
     testRoundTripAuto();
+    testSerializeByteIdentity();
     testDeserializeEmptyIsDefault();
     testDeserializeMalformedIsDefault();