Fix worldspace export baking ancestor transforms twice

[paulgolter]

Problem

Exporting with -worldspace produces wrong transforms when the selected
objects have ancestors that are also exported. This happens in two common
setups:

• -exportSelected with -exportRoots pointing at an ancestor of the
  selection.
• -exportSelected with a node deep in a hierarchy (its parents are
  always exported with their transforms).

In both cases the ancestor transforms end up in the file twice. They are
written on the ancestor prims, and they are also baked into the xform ops
of every selected prim. When the file is composed, the objects move twice
as far, scale twice, and so on.

Repro

# group1 at translate (5, 0, 0), pCube1 under it at translate (0, 2, 0)
cmds.select("pCube1")
cmds.mayaUSDExport(file=path, selection=True, worldspace=True)

The cube lands at world position (10, 2, 0) instead of (5, 2, 0).

Cause

needsWorldspaceTransform in transformWriter.cpp enables the ancestor
bake for every transform found in the export selection (dagPaths). That
is only correct when the selected node becomes a root prim of the file.
The existing worldspace tests only cover exports without a selection,
where dagPaths happens to hold the export roots, so the problem never
showed up there.

Fix

Bake the ancestor chain only into prims that become top-level prims of
the exported file. A prim is top-level when its Maya parent maps to no
prim in the export, either because the node sits under the Maya world
root or because the export roots exclude its parent.

This matches what the existing selNone worldspace tests in
testUsdExportRoots.py already assert: the bake lands on the export
root itself, and prims below it keep their local transforms.

As a side effect, transforms above a non-top-level export root are now
captured on the export root prim instead of being double-applied below
it.

Tests

Two new cases in testUsdExportRoots.py:

• testExportRoot_rootMid_selCube_worldspace — export root above the
  selection. Fails without the fix (double transform), passes with it.
• testExportRoot_rootCube_selCube_worldspace — the selected node is
  itself the export root. Guards that the bake stays where it is needed.

All existing testUsdExportRoots and testUsdExportRootsAndRootPrim
cases pass unchanged.

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🤖 Generated with Claude Code

[pierrebai-adsk]

Good job!