Fix instancingLodFactor never selecting the original mesh

QSSGRenderSubset::lods only holds the simplified levels; the original
geometry is level 0 and is reached by leaving currentLod at -1. The
manual instancing path mapped the factor onto lods.size() - 1 and then
added 1, so instancingLodFactor 0.0 drew the first simplified level and
the full-detail mesh was unreachable for instanced models.

Map the factor over all lods.size() + 1 levels instead, so 0.0 selects
the original mesh and 1.0 the coarsest level, as documented. The
instanced factor 0 model in LodFactorLevel0.qml now matches the
non-instanced reference next to it.

Amends 0a6a58b8c497c6915d3930c620d01451b2dc039e (Add manual LOD factor
for instancing).

Fixes: QTBUG-151352
Pick-to: 6.12
Assisted-by: Claude Code Fable 5.1
Change-Id: I8c4747b7e1c6b69ec4edbfeee504034c5bff3183
Reviewed-by: Andrei Yankovich <EndrIIMail@gmail.com>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>