Saving from a non-first paper layout still reopened on the first paper
tab: set_saved_active_layout only updated an existing CTAB entry, but
documents authored here never carried one, so CTAB was never written and
the reader fell back to $TILEMODE (which only records model-vs-paper) →
the first paper layout.
set_saved_active_layout now creates the CTAB DICTIONARYVAR under the root
named-object dictionary when it is absent (updating in place otherwise).
The root dictionary is taken from the header handle, or found by scanning
for the dictionary that holds ACAD_LAYOUT when a from-scratch document
has not populated the header handle yet. The writers persist it (they
serialize the document's objects and root-dictionary entries; the
root-dict rebuild in CadDocument::build() is not run on save).
Adds tests/active_space_roundtrip.rs, including a full DXF save→reload
round-trip asserting both $TILEMODE and CTAB survive.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
