docs(fix_orientation): deskew / libtiff integration spike notes (#116)

Add DeskewExternalToolSpike.h with an integration outline and list it in the
fix_orientation CMake sources for discoverability.

Author: pablogventura

src/core/filters/fix_orientation/CMakeLists.txt

@@ -5,6 +5,7 @@ set(ui_files
     OptionsWidget.ui)
 
 set(sources
+    DeskewExternalToolSpike.h
     ImageView.cpp ImageView.h
     Filter.cpp Filter.h
     OptionsWidget.cpp OptionsWidget.h

src/core/filters/fix_orientation/DeskewExternalToolSpike.h

@@ -0,0 +1,26 @@
+// Copyright (C) 2026 ScanTailor-Advanced contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+//
+// Spike notes for GitHub #116 (TIFF deskew / external deskew integration).
+// This header is documentation only; it is not wired into the build.
+//
+// Integration outline:
+// 1. Decide scope: batch-only vs interactive, and whether to shell out to a
+//    system tool (e.g. tiffcp/tiffutil) vs link libtiff directly.
+// 2. If external binary: locate executable per platform (Windows/macOS/Linux),
+//    validate on startup or lazily, surface a clear error in the UI.
+// 3. Pipe I/O: write a temporary TIFF, run deskew, read back; or use in-memory
+//    API if linking libtiff — mind thread safety with the worker pool.
+// 4. Preserve metadata (DPI, photometric) and failure modes (OOM, corrupt TIFF).
+// 5. Add an optional CMake feature flag so packagers can disable the dependency.
+//
+// Prefer keeping experimental code on a branch until packaging and UX are settled.
+
+#ifndef SCANTAILOR_FIX_ORIENTATION_DESKEW_EXTERNAL_TOOL_SPIKE_H_
+#define SCANTAILOR_FIX_ORIENTATION_DESKEW_EXTERNAL_TOOL_SPIKE_H_
+
+namespace fix_orientation {
+// Intentionally empty: see file comment above.
+}  // namespace fix_orientation
+
+#endif