[MAINTENANCE] Remove archive/ directory and dead placeholder code

[yunlishao-vibe]

Task Summary

Remove dead placeholder code that accumulates technical debt: binary artifacts committed to CommonLib, empty test stub directories, and tmp/sumo_build/ which may be tracked depending on local state.

Note: archive/ is already covered by .gitignore (**/archive/*), so its contents are not tracked in git. The directory exists locally but need not be removed from git — no action needed there.

Why is this needed?

• Binary build artifacts (*.lib, *.mexw64, *.a) are committed directly in CommonLib/ — these belong in build/ or should be regenerated by the build system
• tests/Applications/ contains only a single empty stub directory Eco_Fixed_Timming/ and a README with no working tests
• tests/UserScenarios/ is completely empty
• tmp/ is gitignored via **/tmp/* but the pattern may not cover all cases
• CommonLib/ryml_all.hpp is a 1.1MB vendored single-header library that was likely only used during a migration and may no longer be needed

Scope of Work

1. Audit and remove committed binary artifacts

• CommonLib/*.mexw64, CommonLib/*.lib, CommonLib/*.a — check if these need to stay in git or can be built/downloaded
• If they must stay (for prebuilt distribution), move to build/ with clear README
• Update .gitignore to prevent accidental re-commit of build artifacts

2. Clean up empty/stub test directories

• tests/Applications/Eco_Fixed_Timming/ — empty stub, remove or replace with working test
• tests/UserScenarios/ — empty, either remove or add README.md explaining purpose
• tests/Applications/README.md — update to reflect actual content

3. Audit ryml_all.hpp

• Check if CommonLib/ryml_all.hpp is used anywhere in the codebase
• If not used, remove (1.1MB savings)
• If needed, add to dependencies.yaml and document its purpose

4. Add auto-test

• Add tests/Python/test_repo_hygiene.py that validates:
  • No .mexw64, .lib, .a binaries in CommonLib/ source tree (only in build/)
  • All directories under tests/ contain at least one non-README file
  • ryml_all.hpp is either documented or absent

Branch and PR Target

• All PRs for this issue should target the dev branch (not main)
• First ensure main is merged into dev before starting work

Files Involved

• CommonLib/ (binary artifacts)
• CommonLib/ryml_all.hpp
• tests/Applications/
• tests/UserScenarios/
• .gitignore
• tests/Python/test_repo_hygiene.py (new)

Acceptance Criteria

• No unneeded binary artifacts committed in CommonLib/ source tree
• ryml_all.hpp removed or documented
• Empty test stub dirs removed or filled
• tests/Python/test_repo_hygiene.py passes
• .gitignore updated to prevent recurrence