Move roadmap backlog out of README

ROADMAP.md

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+# pdf-tools Roadmap
+
+Active roadmap items are tracked as Gitea issues. This file is kept as the source for issue import and wiki sync; completed items remain here as release context.
+
+## Milestone 1: Fast preview and thumbnails
+
+- [x] Remove unused legacy page list view.
+- [x] Bound thumbnail generation by width and height.
+- [x] Display thumbnails progressively.
+- [x] Add preview page flipping.
+- [x] Attach preview controls to the modal container.
+- [x] Add first keyboard shortcuts.
+- [x] Cache thumbnails by page and rotation.
+- [x] Regenerate only changed rotated thumbnails.
+- [x] Show software version number.
+
+## Milestone 2: Real page workspace
+
+- [x] Introduce stable page references instead of only original page indices.
+- [x] Support duplicate selected pages.
+- [x] Save and reload the last state from browser storage.
+- [x] Support named workspaces.
+- [x] Store PDF binaries directly in IndexedDB.
+- [x] Reset workspace.
+- [x] Add command history as a foundation for undo/redo.
+- [x] Add undo/redo.
+- [x] Display undo/redo history with redo entries visually separated.
+- [x] Extract selection as a new active workspace.
+- [ ] Reduce undo/redo storage footprint if large documents make snapshots too heavy.
+- [ ] Add grid/list view toggle.
+
+## Milestone 3: Better merge and mobile handling
+
+- [x] Add a full multi-file merge queue.
+- [ ] Support drag-and-drop of PDFs into the page grid at the hovered position.
+- [ ] Add custom long-press drag on mobile.
+- [ ] Consolidate frequently used actions into a toolbar.
+
+## Milestone 4: Structural PDF editing
+
+- [ ] Metadata editing.
+- [ ] Crop pages.
+- [ ] Add tools directly in the preview overlay.
+- [ ] Read/fill/flatten forms.
+- [ ] Read bookmarks, then evaluate bookmark editing.
+- [ ] Read annotations, then evaluate annotation writing.
+
+## Milestone 5: Export and power tools
+
+- [ ] Basic text extraction.
+- [x] ZIP export for split results.
+- [ ] Optimize/compress MVP.
+- [ ] Carefully scoped encrypted PDF handling.
+
+## Non-goals for now
+
+- Server-side PDF processing.
+- Collaborative editing.
+- User accounts.
+- OCR.
+- Full content-stream editing.
+- Digital signature creation/validation workflows.
+- DMS replacement functionality.