Props ignore @pitch and @roll everywhere they are drawn or exported

[JArmandoAnaya]

Found while implementing p6-s9 (#323); deliberately left out of that sprint
rather than widening its blast radius into the render and export paths. The
evidence is recorded on #323
(note 3).

The gap

Object::pitch and Object::roll (§13.1, Table 85) are parsed, written and
round-tripped correctly — and then silently ignored by the mesher and by
both exporters, for every prop in the product, not just for assembly parts.

The cause is one struct: mesh::ObjectInstance
(core/include/roadmaker/mesh/mesh.hpp:145-154) carries a position, a heading
about +Z, and one uniform scale. There is nowhere for a pitch or a roll to go,
so mesh_builder never reads them and the glTF and USD exporters never see them.

Why it is worth fixing rather than documenting

• A file authored elsewhere that leans a sign back 5° opens in RoadMaker looking
  upright, saves back byte-identical, and the user never learns the viewport is
  lying to them. Silent disagreement between what is stored and what is drawn is
  exactly the class of bug the never-drop contract exists to prevent.
• It is the reason the bundled signal_mast assembly ships its arm as
  already-horizontal geometry rather than as an upright post rotated 90°
  (p6-s9). A part that must lie down has to arrive lying down, which constrains
  how every future assembly is authored.

Scope

• Widen mesh::ObjectInstance to a full orientation (the minimal change is two
  more angles; a 3×3 or a quaternion is the alternative — worth deciding rather
  than defaulting).
• Read @pitch/@roll in mesh_builder's object placement.
• Carry the orientation through the glTF and USD exporters, and through the
  instanced render fast path (p6-s6) — the batched path is where a naive fix
  regresses performance, so measure.
• The properties panel gains the two fields once they mean something.
• Once it lands, props::AssemblyPart can gain dpitch/droll and the mast arm
  can become an ordinary post with a rotation.

Acceptance

• A corpus file with a non-zero @pitch on a prop renders leaning, and exports
  leaning to both glTF and USD.
• The instanced path draws the same orientation as the non-instanced one.
• Round-trip stays byte-identical (this is a render/export fix, not a
  persistence change — persistence already works).