test(264): pin the path-scan-error log; correct testing.md accuracy

Re-review of the previous fix commit (986ccfaf, verdict MERGEABLE)
raised one fair hit and one claim that did not survive checking.

Fair hit — that commit's message asserted "LogError is an extension
method that NSubstitute can't cleanly verify" as the reason for adding
no test. That is FALSE, and this repo disproves it:
ErsatzTV.Tests' ReleaseMemoryHandlerTests.ShouldHaveLogged asserts on
ILogger via ReceivedCalls(), which works precisely because the LogError
extension dispatches to the substituted ILogger.Log. Reusing that idiom
here costs 4 lines, so pin the log the previous commit added. Verified
non-vacuous: asserting a message the handler never logs fails the test.

Not applied — the same review called the row's "1471 tests" misleading
on the grounds that TranscodingTests is [Explicit] and contributes most
of that count. TranscodingTests is indeed [Explicit], but filtering it
out yields exactly 1471, so 1471 is already the runnable count and the
row was accurate. Kept the number; documented TranscodingTests as
[Explicit]/opt-in instead, since it was a genuine omission from a table
that claims to be authoritative.

Also rewrap the verification-gate paragraph the previous commit left
over-long, and name the two always-run projects instead of "both".

Deferred (filed separately): a canceled local scan now logs at ERROR
per path via this log. It mirrors the remote handlers exactly, so
diverging here would be the inconsistency, not the fix.

ErsatzTV.Scanner.Tests/Application/MediaSources/ScanLocalLibraryHandlerTests.cs

@@ -27,6 +27,7 @@ public class ScanLocalLibraryHandlerTests
             _libraryRepository = Substitute.For<ILibraryRepository>();
             _configElementRepository = Substitute.For<IConfigElementRepository>();
             _movieFolderScanner = Substitute.For<IMovieFolderScanner>();
+            _logger = Substitute.For<ILogger<ScanLocalLibraryHandler>>();
 
             _library = new LocalLibrary
             {
@@ -57,6 +58,7 @@ public class ScanLocalLibraryHandlerTests
         private ILibraryRepository _libraryRepository;
         private IConfigElementRepository _configElementRepository;
         private IMovieFolderScanner _movieFolderScanner;
+        private ILogger<ScanLocalLibraryHandler> _logger;
         private LocalLibrary _library;
 
         [Test]
@@ -88,6 +90,10 @@ public class ScanLocalLibraryHandlerTests
 
             _library.LastScan.ShouldBeNull();
             await _libraryRepository.DidNotReceive().UpdateLastScan(Arg.Any<Library>());
+
+            // a failed path suppresses the library-level scan time, reproducing the #264 symptom;
+            // without this log the user would have nothing explaining why
+            ShouldHaveLogged("Error scanning local library path /movies: scan failed");
         }
 
         [Test]
@@ -168,6 +174,13 @@ public class ScanLocalLibraryHandlerTests
                 Substitute.For<ISongFolderScanner>(),
                 Substitute.For<IImageFolderScanner>(),
                 Substitute.For<IRemoteStreamFolderScanner>(),
-                Substitute.For<ILogger<ScanLocalLibraryHandler>>());
+                _logger);
+
+        // asserts on the substituted ILogger.Log call the LogError extension dispatches to
+        // (same idiom as ErsatzTV.Tests' ReleaseMemoryHandlerTests.ShouldHaveLogged)
+        private void ShouldHaveLogged(string expectedMessage) =>
+            _logger.ReceivedCalls()
+                .Any(call => call.GetArguments().ElementAtOrDefault(2)?.ToString() == expectedMessage)
+                .ShouldBeTrue();
     }
 }