Collapse the duplicated enum tables and XML helpers the persistence layer maintains twice (ponytail audit)

[JArmandoAnaya]

A repo-wide over-engineering audit (244k tracked source lines, all 13
FetchContent deps, 9 CMake options, 14 CI jobs) found the tree broadly
disciplined: no dead public API (all 262 RM_API functions have a
non-test caller), no unused dependency, no dead build flag, one
orphaned tracked image.

What it did find is a cluster of deliberate duplication, most of it in the
persistence layer: the OpenDRIVE and OpenSCENARIO reader/writer pairs each keep
their own copy of the same XML helpers and their own one-directional
enum↔string table — several with comments explicitly defending the copy
against a divergence that has not happened in the project's lifetime.

That duplication is not cosmetic. #476 was exactly this class of defect —
the writer rewrote lane/mark enums into different semantics than the reader
parsed, because the two tables were maintained separately. The probe on that
issue found 9 corruptions where the issue body listed 3. Collapsing each pair
to one table makes the class unrepresentable rather than merely fixed.

Scope — two sequential PRs, no behavior change

PR-A — kernel dedup (≈ -235 lines)

	what	where
A1	One constexpr table per enum, serving both directions, replacing 6 reader if-chains + 6 writer switches	new core/src/xodr/enum_names.hpp
A2	One copy of to_double / num / set_num / set_optional_num / append_fragment / node_to_string / trim	new core/src/xml/xml_common.hpp
A3	linked_end is byte-identical in two files	→ core/src/road/junction_adjacency.hpp
A4	is_one_of hand-rolls a loop the stdlib ships	→ std::ranges::find
A5	Environment::procedural_sky is set, asserted by two tests, and never read by GLRenderer	delete

Three invariants A1 must not violate, each preserving today's behaviour:

1. Write-fallbacks are not table rows. LaneType::Other → "none",
   ObjectType::Other → "none", RoadMarkType::Other → "solid" are the
   writer's fallback argument. As table rows they would make the reader map
   "solid" → RoadMarkType::Other — the fastest way to reintroduce xodr: unknown enum spellings are rewritten on save — lane type → "none", road-mark type → "solid", colour → "standard" #476.
2. Empty-string policies stay at the call site. value_of returns
   nullopt for unknown; the reader keeps its own Other/nullopt policy.
3. type_str preservation is untouched — the writer already prefers the
   verbatim spelling over the enum name.

PR-B — docs-site (≈ -175 lines)

• B1 Delete docs-site/scripts/report-external-links.mjs — 125 lines of
  hand-rolled fetch + concurrency + timeout for a report that always exits 0.
  lychee is already a CI dependency; a weekly non-blocking lychee job without
  --offline replaces it in ~15 lines of YAML.
• B2 Delete docs-site/scripts/theme-css.mjs — a JavaScript parser of C++
  source (indexOf + a QColor(0x..) regex over theme.cpp) that breaks on
  any reformat. The repo already solved this for editor/resources/help/help.css:
  generate in C++, commit the output, byte-gate it from a gtest. Reuse that.

Acceptance — byte-identical output, not green tests

Tests can pass while a round trip silently changes bytes; that is how #476
survived. A throwaway differential probe reads and rewrites all 126 tracked
.xodr files plus tests/esmini/*.xosc on main and on the branch, and
diffs the bytes. Zero differences is the gate, and the gate itself is
sabotage-checked (swap two spellings in the table; the probe must go red).

Declined by the maintainer, deliberately out of scope

• Cutting the Renderer interface (10 pure virtuals, one implementation,
  no test double). It is a documented architecture non-negotiable in the
  project guide; kept.
• Recompressing the 13 MB of docs/design/ui-revamp/ PNGs. Git history
  already carries the original bytes, so recompressing grows the repo net
  unless history is rewritten. Not worth it for design screenshots.
• point_segment_distance (identical in geometry/simplify.cpp and
  mesh/junction_corner_detail.cpp) — the two subsystems share no internal
  header, and creating one to save 13 lines is break-even.
• The PhaseSemantics read/write twin — one enum, six values, ~30 lines.
  Not worth a new header.

Net: ≈ -410 lines, 0 dependencies removed (every one of the 13 is
load-bearing; sharp is already stubbed for licence reasons).

[JArmandoAnaya]

Both PRs merged: #564 (kernel dedup, c33314c) and #565 (docs-site, 419dd8c). 34 CI checks across the two, all green, zero fix attempts.

Net −155 lines of executable code (−99 kernel, −56 docs-site). Two of the audit's nine findings were declined by the maintainer and are recorded as out of scope in the issue body; two more I scoped out myself as break-even (point_segment_distance, the PhaseSemantics twin).

The thing worth remembering is that my first acceptance gate was blind. The differential probe — read and re-emit all 143 tracked .xodr/.xosc fixtures, diff the bytes — reported zero differences against a deliberately corrupted table, twice over: a round trip is invariant under a consistent permutation, and *::type_str means a parsed file never consults these tables at all. The obvious unit test (value_of(name_of(v)) == v) is a tautology for the same reason. core/tests/test_enum_names.cpp pins the spellings against literals transcribed from the ASAM tables instead, and catches all four sabotages the probe called clean.