Save a .qetz without building the folios not touched (stage D4)

With QET_LAZY_FOLIOS=1, a project opened from a .qetz's index saves a
.qetz without building the folios it never built, when building them
would write the same: their parts are copied as they were and their rows
of the document's engineering data from the project.sqlite they were read
from (QetContainerDb::extract() with kept_from, QetContainer::split() with
the old entries, pictures included). Whether a folio may be kept follows
the rules of stage 5.5 (QETProject::toXml() with kept_parts): nothing
project-wide changed since opening; a folio whose own data changed is
built with its partners; after an edit, folios holding a table filled
from the database are built; the partners still waiting of every built
folio are built. Trusted only for a project opened from a format 3 index
written by this version whose folios are named by their uuid.

Format 3 names a folio by its uuid in the database (doc_meta folio_ids =
uuid) when every folio has one of its own, so that a kept folio keeps its
rows whatever its place; copies keep the rows' order (ORDER BY rowid).
A folio not built keeps the name of its part until it is built
(Diagram::deferredPart()).

Measured, the 136-folio f2099 opened from its .qetz and saved as a .qetz:
4.75 s with no folio built, 33 s with every folio built; its 136 folio
parts copied byte for byte; a kept save of a kept save is the same bytes.
industrial.qetz untouched: 28 of its 47 folios not built are built to save
(link chains, as in 5.5); after everyday edits on its busiest folio, 7
stay not built.

tst_lazyfolios: an untouched .qetz saved without building, read back,
holds the project; edits saved without building hold, read back, what the
same edits saved with every folio built hold (read back too: a wire
numbered from its potential is saved with no sequential numbers whichever
the way -- reading it back renumbers it, a separate issue); fails when a
kept folio's symbol information is not copied. Corpus, 342 projects: a
.qetz saved without building, read and saved as .qet = eager = master on
all 342. ctest
96/96 with and without QET_LAZY_FOLIOS=1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/diagram.cpp

@@ -798,9 +798,6 @@ QDomElement Diagram::deferredXml()
 {
 	if (m_deferred.isNull() && !m_deferred_part.isEmpty() && m_project) {
 		m_deferred = m_project->readFolioPart(m_deferred_part);
-		if (!m_deferred.isNull()) {
-			m_deferred_part.clear();
-		}
 	}
 	return m_deferred;
 }