Data integrity and round-trip fidelity for DXF/DWG files, preserving unknown objects, xdata, extension dictionaries, and various CAD elements during save/reopen cycles.

## Summary
"Saving drops whatever we don't model" (`docs/gaps.md` Top-10 #1 and FF1, an alpha blocker) has been partly fixed by #201, #204, #208, #209, #213, #223, #147 and #162. Open PRs add MULTILEADER (#459), groups (#417), VIEW/VPORT tables (#463) and SORTENTSTABLE (#439). The row no longer says what is still lost.

## Scope
- Audit, with synthetic DXF files built from the public DXF Reference, what survives open → save → reopen on current main, for:
- unknown entities in model space, paper space and blocks;
- unknown objects in the named object dictionary and nested dictionaries;
- foreign xdata (other APPIDs) on entities, tables and blocks;
- extension dictionaries (102 {ACAD_XDICTIONARY) on entities and table records;
- IMAGE + IMAGEDEF + IMAGEDEF_REACTOR;
- UCS table, layer states, block descriptions;
- true colours and colour books on every object type;
- transparency;
- hyperlinks (xdata PE_URL);
- DICTIONARYVAR, XRECORDs, persistent reactors.
- Fix what is still lost, keeping data as raw groups where we don't model it, with handles remapped consistently. Owner/reactor handles must point at real objects after save.
- Do the same through the DWG bridge where the bridge allows it, and list acadrust gaps.
- Update the Top-10 #1 and FF1 rows to the true state.

## Expected
Opening a colleague's file and saving it keeps everything CADCraft doesn't edit.