Document performance scalability

[appleweiping]

Summary

• Add a dedicated scalability section to the performance guide.
• State the qualitative scalability conclusion requested in [IMPROVEMENT] Update performance guide with scalability #1096, without reintroducing the superseded quantitative analysis.

Validation

• npx --no-install markdownlint docs/user_manual/performance-guide.md
• .venv\Scripts\reuse.exe lint
• git diff --check upstream/main...HEAD
• sphinx-build -W --keep-going -b html docs docs\_build\html was attempted locally; the build could not complete because the generated Doxygen XML file docs/doxygen/build/xml/index.xml is absent in the local environment. The hosted documentation check will provide full Sphinx validation.

AI assistance

OpenAI Codex assisted with repository navigation, scope verification, drafting, and validation. I reviewed and understand the change and remain responsible for the contribution.

Assisted-by: OpenAI Codex

Closes #1096

[appleweiping]

Maintainer note: GitHub does not grant my account permission to assign this PR or apply labels, and the repository workflows (CI Build, Sonar Cloud, and Check PR Labels) are still awaiting maintainer approval. Could a maintainer please assign @appleweiping, add the appropriate label (likely improvement), and approve the workflows when convenient? DCO and Read the Docs are already passing. Thank you.