fix(xodr): preserve the five warned-and-dropped spec areas (#539)

[JArmandoAnaya]

The defect

Five spec areas were warned about and then thrown away — one diagnostic,
then permanent data loss:

Element	Spec	What a save destroyed
<lateralProfile><shape>, legacy <crossfall>	§10.5.1	flattened a non-planar carriageway
road <surface>/<CRG>	§10.6	the referenced surface detail
<junctionGroup>	§12.16	the standard's own roundabout grouping (#495)
<railroad>, root <station>	chapter 15	a tram file's rail layer
<include> outside <header>	§7.1	the include itself

Loud-and-lossy is worse than silent in one respect: the file looked like it had
been understood.

What landed

All five ride the preserved tier built for #453 — reusing
capture_unmodeled() rather than adding five more walks. The change is a set
of modeled-member lists plus three writer emission points; that reuse is exactly
what fmt-f1 was shaped for.

Three new homes for the fragments: Road::lateral_profile_extras,
Road::road_extras, and RoadNetwork::preserved_root_children() (a sibling of
preserved_user_data(), separate because the writer emits the two in different
places).

The diagnostic downgrade is half the fix. "is not supported yet and was
ignored" was true before and is a lie now. note_preserved() says "is
preserved verbatim but not modeled; it round-trips unchanged and has no effect
in this build" — the user needs the second half without being told the first,
since a preserved <shape> still will not bend the carriageway.

Nothing here is modeled, and none of it needs to be for v0.1.0. Modeling stays
with whoever owns the feature later — <junctionGroup> with #495.

Tests

New corpus seed preserved_sweep.xodr carrying all five areas.

• TheWarnedAndDroppedScopesNowSurviveWriteParseWrite — all eight markers in
  the emitted bytes, plus the content (file="pavement.crg",
  junction="101", file="extra_geometry.xodr"): a writer emitting an empty
  <shape/> would satisfy a tag-only search while still having flattened the
  carriageway. Then the write→parse→write fixed point.
• TheSweptScopesSayPreservedRatherThanIgnored — asserts the new wording
  appears and that the old "was ignored" claim does not. A fix that
  preserved the data while still saying it was discarded would pass a
  preservation-only check.

Sabotage-verified: reverting root children to warn-and-drop failed both.

Verification

• ctest --preset ci-macos — 3281/3281 pass
• pytest python/tests — 541 passed, 1 skipped
• clang-format clean

Fixes #539