chore(test): clean dead fixture-cache substring reference (#100)

Round-2 follow-up identified by /idd-verify round-2 logic Agent: the
fixture-type detection in NoteHTMLConvertTests.swift still referenced
the removed `macdoc-note-fixture-cache` substring. Detection still
worked because the OR branch on `synthetic-fixture` matched, but the
first branch was dead code.

Replaced with the canonical synthetic-fixture marker per the new
no-cache resolution path: `macdoc-synthetic-fixture-<UUID>`. No
behavior change — same fixture-type detection outcome.

Refs #100

Author: kiki830621

Tests/MacDocCLITests/NoteHTMLConvertTests.swift

@@ -57,8 +57,10 @@ final class NoteHTMLConvertTests: XCTestCase {
         //   "media ≥ 1 asset" + ">750 KB index.html" floors per #81 calibration.
         // - Synthetic fixtures (NoteFixtureGenerator output, per #100) have no
         //   media + minimal strokes → only structural assertions apply.
-        let isSyntheticFixture = fixture.path.contains("macdoc-note-fixture-cache")
-            || fixture.lastPathComponent.contains("synthetic-fixture")
+        // Detection: synthetic fixtures live in $TMPDIR with the
+        // `macdoc-synthetic-fixture-` prefix (per CLITestHelper.swift, no
+        // cache subdirectory since #100 round-2 fix).
+        let isSyntheticFixture = fixture.lastPathComponent.contains("macdoc-synthetic-fixture")
 
         let mediaFiles = try FileManager.default.contentsOfDirectory(
             at: mediaURL,