fix(comments): correct canary justification and expand failure guidance in padding tests

tests/test_app_version_padding.cpp

@@ -2,11 +2,10 @@
 // string threading (Phase V, V1 leading-zero fidelity). No REAPER, no framework.
 //
 // WHY THIS EXECUTABLE EXISTS: test_app_version.cpp pins stampVersion() relative to the
-// live configure_file'd version string — but a live-version assertion can only DETECT a
-// "reconstruct the string from numeric components" regression when the live version
-// happens to be zero-padded. "0.9.01" reconstructed via integers renders "0.9.1" and
-// trips it; "0.9.8" reconstructs to itself, so at an unpadded version the check passes
-// whether or not the code regressed — the guard silently goes vacuous while green.
+// live configure_file'd version string — but a relative assertion (e.g. appVersion() ==
+// stampVersion()) is version-shape-blind: it holds whether the string was threaded
+// verbatim or reconstructed from numeric components, at any version, padded or not.
+// The live suite therefore cannot detect a reconstruct-from-components regression at all.
 // Padded and unpadded versions are both legitimate (Daniel's ruling), so the guard must
 // not depend on the shipped version's shape.
 //
@@ -67,9 +66,11 @@ static void testCanaryFixtureIsNonVacuous() {
         std::to_string(wv.parsed.major) + "." +
         std::to_string(wv.parsed.minor) + "." +
         std::to_string(wv.parsed.patch);
-    // If this fails, the fixture is unpadded and the canary guards nothing.
-    // Restore a zero-padded fixture (e.g. "0.9.01") in CMakeLists.txt and
-    // update the matching literals in this file.
+    // If this fails, one of two causes: (1) the fixture was changed to an unpadded form
+    // (e.g. "0.9.1") — restore a zero-padded fixture (e.g. "0.9.01") in CMakeLists.txt
+    // and update the matching literals in this file; or (2) the fixture is still padded
+    // but app_version.cpp regressed to parse-and-reprint — in that case
+    // testPaddedVersionSurvivesThreadingVerbatim also fails, pointing at the real cause.
     CHECK(reprinted != stampVersion());
 }