fix(core): File Provider write probe — OneDriveLocator.probeWritable(at:)

isWritableDirectory (permissions bits) is not enough: a OneDrive
Files-On-Demand directory whose provider domain is signed out can report as
existing and POSIX-writable while an actual write fails. probeWritable
writes a small ".redline-probe-<uuid>" file into the folder via
AtomicFile.write (open+write+fsync+rename+directory-fsync), then removes it;
any failure at write, fsync, or removal means false.

Three unit tests: an ordinary writable directory (true, and no probe file
left behind), a chmod 500 directory (false; permissions restored in
teardown), and a plain file path (false).

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

Sources/ShotdeckCore/Support/TransportSettings.swift

@@ -176,4 +176,29 @@ public enum OneDriveLocator {
         guard exists, isDirectory.boolValue else { return false }
         return fileManager.isWritableFile(atPath: url.path)
     }
+
+    /// Writes a tiny probe file into `folder`, fsyncs it, then removes it — the only
+    /// reliable way to catch a OneDrive Files-On-Demand directory whose provider domain
+    /// is signed out: such a directory can report as existing and POSIX-writable
+    /// (`isWritableDirectory` returns true) while an actual write fails. True only when
+    /// the write, fsync, AND removal of the probe file all succeed; any failure at any
+    /// of those steps means false, so the caller treats the folder as unavailable
+    /// rather than proceeding to compose a real PDF into it.
+    public static func probeWritable(
+        at folder: URL,
+        fileManager: FileManager = .default
+    ) -> Bool {
+        let probeURL = folder.appendingPathComponent(".redline-probe-\(UUID().uuidString)")
+        do {
+            try AtomicFile.write(Data(), to: probeURL)
+        } catch {
+            return false
+        }
+        do {
+            try fileManager.removeItem(at: probeURL)
+        } catch {
+            return false
+        }
+        return true
+    }
 }