fix(svm): read the map mode's unit, and compose the relative one

[andiwand]

🤖 Generated with Claude Code

Stage 8 of #772, closing its defect 6, stacked on #789 → #788 → #787 → #786 →
#785 → #784 → #779 — review those first; this branch's base is
feat/svm-fills.

The map mode was taken for its origin and scale and nothing else. Two things
follow from that:

• The unit was ignored, so a drawing that switches to twips or points came
  out at the wrong size — off by 1.76 for twips.
• A relative map mode replaced the current one instead of composing with
  it.

MetaMapModeAction::Execute calls OutputDevice::SetMapMode, which replaces
the map mode except where the new unit is MapUnit::MapRelative, and
ImplMapRes::CalcMapResolution says exactly how it composes then:

offset = offset / relative_scale + relative_origin
scale  = relative_scale * scale

The state now holds the resolved mapping — an origin and a scale — rather
than the map mode as read, and the scale carries the unit: the drawing is
measured in the header's unit, so a map mode in another one scales by what the
two are worth. MapPixel and the two font units are device-dependent, and a
metafile has no device, so they resolve at the same 96 dpi a bitmap without a
size of its own is drawn at.

Nothing in the corpus moves

4 of the 1125 harvested metafiles carry a MAPMODE action at all, and the one
relative map mode among them (in odr-public/svm/table-1.svm) composes to
exactly what replacing it produced, because its scale is 1:1. The reference
output does not change and there is no pin bump.

LibreOffice is not the oracle here

Asked to convert a metafile that switches to twips and then to a relative map
mode, LibreOffice's own svg export drops the third rectangle and writes the
second one outside the placement box. Its metafile player is right; the
exporter is not, so this follows ImplMapRes from source. The one number the
export does agree on is the twips conversion — it writes the rectangle at
1058 wide 884, and so do we.

Verification

4 new tests (44 in the svm suite): a unit switch scales, a relative map mode
composes its origin, a relative map mode multiplies its scale, and a pop
restores the mapping.