Docs fail with Documenter 1.18 on DMDSVD cross-reference

Summary

The documentation build on clean master at 0c0e97f762713be03e5854e853a4c5ec7fd66411 fails after resolving Documenter 1.18.0. Documenter cannot resolve the existing DMDSVD reference in docs/src/libs/datadrivendmd/koopman.md.

Reproduction

$ TMPDIR=/home/crackauc/tmp julia +release --project=docs docs/make.jl
[ Info: CrossReferences: building cross-references.
┌ Error: Cannot resolve @ref for md"[DMDSVD](@ref)" in docs/src/libs/datadrivendmd/koopman.md.
│ - Header or `@id` anchor with slug 'DMDSVD' in docs/src/libs/datadrivendmd/koopman.md does not exist.
│ - Header or `@id` anchor with slug 'DMDSVD' is not unique in docs/src/libs/datadrivendmd/koopman.md.
└ @ Documenter ~/.julia/packages/Documenter/V8kkd/src/utilities/utilities.jl:47
ERROR: LoadError: `makedocs` encountered an error [:cross_references] -- terminating build before rendering.

The same clean checkout passes after pinning Documenter 1.17.0:

$ julia +release --project=docs -e 'using Pkg; Pkg.pin(PackageSpec(name = "Documenter", version = v"1.17.0"))'
$ TMPDIR=/home/crackauc/tmp julia +release --project=docs docs/make.jl
[ Info: CrossReferences: building cross-references.
[ Info: CheckDocument: running document checks.
[ Info: RenderDocument: rendering document.
[ Info: HTMLWriter: rendering HTML pages.
[ Info: Automatic `version="1.16.0"` for inventory from ../Project.toml

The latest successful master documentation run used Documenter 1.17.0: https://github.com/SciML/DataDrivenDiffEq.jl/actions/runs/33310090858

Environment

Julia Version 1.12.6
Commit 15346901f00 (2026-04-09 19:20 UTC)
Official https://julialang.org release
Linux (x86_64-linux-gnu)
CPU: AMD EPYC 7502 32-Core Processor
LLVM: libLLVM-18.1.7 (ORCJIT, znver2)

🤖 Generated with Codex CLI 0.151.0 (model: gpt-5).
Session: local Codex session ID 01a0588b-a3c5-7040-8363-097859b8f435.

Run the documented julia +release --project=docs docs/make.jl command and inspect docs/src/libs/datadrivendmd/koopman.md, focusing on the DMDSVD cross-reference. Compare the build with Documenter 1.17.0 and 1.18.0; done means the docs build completes with the newer version and resolves the reference without an error.