fix(xodr): round-trip virtual-junction linkage and connection internals (#537)

[JArmandoAnaya]

Three fidelity holes, all in modeled scopes that #453's preservation sweep
deliberately does not cover. Built on #534's junction test scaffolding, as the
issue intended.

1. @elementS/@elementDir were read nowhere

asam.net:xodr:1.7.0:road.linkage.virtjunc_link_attribute_usage requires
both on a <link> pointing at a virtual junction. read_link took only
elementType/elementId/contactPoint, with no diagnostic, and
write_link_element never emitted them — so a spec-conformant file lost its
virtual-junction linkage on the first save.

They ride RoadLink now (element_s as an optional, element_dir verbatim —
it is a two-value enumeration whose spelling is the whole content) and are
written back only when the source carried them. An ordinary road link, where
the standard forbids them, stays byte-identical; there is a test for exactly
that, because "write them always" would churn every fixture in the suite and
assert something the spec disallows.

2. A virtual junction's own <connection>s were deleted

§12.7 permits them, but the arms-xor-spans policy cleared the list — warned, and
then destroyed the data.

They are preserved now, in Junction::preserved_connections: held out of
connections so nothing tries to build geometry from them, and re-emitted
unchanged. Worth being precise about why that is not a contradiction —
arms-xor-spans is about generation. A span junction never cuts its main
road, so there is nothing to derive; that stays true. What was wrong was
concluding the file's data must therefore be discarded.

3. <connection> @type and non-<laneLink> children were dropped

The reader iterated children("laneLink") only. @id regeneration is benign —
the writer renumbers connections deterministically — but @type (§12.3
Table 61) carries meaning the file owns, and anything a later revision adds was
vanishing too.

Tests

New corpus seed virtual_junction_links.xodr: a road linking into a virtual
junction with @elementS/@elementDir on both ends, and a junction
declaring a connection that carries @type and an unmodeled child — so cases 2
and 3 ride together.

VirtualJunctionLinkageAndConnectionsSurviveWriteParseWrite asserts the model,
then the bytes (a model-only check passes on a writer that emits none of
it), then the fixed point. AnOrdinaryRoadLinkWritesNoElementSOrDir pins the
byte-stability of every other link.

Sabotage-verified: re-deleting the virtual junction's connections failed the
round-trip test.

Verification

• ctest --preset ci-macos — 3283/3283 pass
• pytest python/tests — 541 passed, 1 skipped
• clang-format clean

Fixes #537