test: launch-paths BLOCKER regression + isWritableDirectory unit tests

ReturnWatcherTests.swift: two new tests bracket the BLOCKER fix — one
characterizes the old bug (FolderSettings.resolvedAppSupportPaths(), AirDrop-
only, ignores the persisted OneDrive transport; the watcher ends up watching
a stale isolated folder and misses a marked PDF dropped into the real
OneDrive-mode folder), the other proves the fix (the exact launch/bootstrap
construction — TransportSettings.resolvedAppSupportPaths() +
watcher.updateWatchFolder() before start — detects it). Both isolated to
temp dirs, including an explicit FolderSettings watch-folder override so the
"bug" test's found.isEmpty assertion never depends on what's actually in
Ben's real ~/Downloads (it does, in fact, already contain real marked-up
Redline PDFs from prior testing — an earlier version of this test read the
REAL Downloads folder and failed for exactly that reason).

TransportSettingsTests.swift: three tests for
OneDriveLocator.isWritableDirectory — true for an ordinary directory, false
for one chmod'd 500 (permissions restored in teardown before removal), false
for a plain file and for a nonexistent path.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Tests/ShotdeckCoreTests/TransportSettingsTests.swift

@@ -90,6 +90,38 @@ func oneDriveFolderUnavailableErrorDescriptionContainsThePath() throws {
     #expect(description.contains(path))
 }
 
+@Test
+func isWritableDirectoryTrueForAnOrdinaryWritableDirectory() throws {
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-writable")
+    defer { try? FileManager.default.removeItem(at: dir) }
+    #expect(OneDriveLocator.isWritableDirectory(at: dir))
+}
+
+@Test
+func isWritableDirectoryFalseForAnExistingButUnwritableDirectory() throws {
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-unwritable")
+    defer {
+        // Restore perms BEFORE removal — an unwritable dir can't otherwise be cleaned up.
+        try? FileManager.default.setAttributes([.posixPermissions: 0o755], ofItemAtPath: dir.path)
+        try? FileManager.default.removeItem(at: dir)
+    }
+    #expect(OneDriveLocator.isWritableDirectory(at: dir))   // sanity check before chmod
+
+    try FileManager.default.setAttributes([.posixPermissions: 0o500], ofItemAtPath: dir.path)
+    #expect(!OneDriveLocator.isWritableDirectory(at: dir))
+}
+
+@Test
+func isWritableDirectoryFalseForAPlainFileAndForANonexistentPath() throws {
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-writable-check-parent")
+    defer { try? FileManager.default.removeItem(at: dir) }
+    let filePath = dir.appendingPathComponent("plain-file.txt")
+    FileManager.default.createFile(atPath: filePath.path, contents: Data("x".utf8))
+
+    #expect(!OneDriveLocator.isWritableDirectory(at: filePath))
+    #expect(!OneDriveLocator.isWritableDirectory(at: dir.appendingPathComponent("does-not-exist")))
+}
+
 struct TransportDefaultsSuite {
     let name: String
     let defaults: UserDefaults