test(app): cover the static and source asset lookups

[aaaaahaaaaa]

What

interloper-app's Python package had no test suite at all, leaving its two asset-path lookups at 65% (static_dir and source_dir error paths uncovered). This brings the package to 100%.

	before	after
interloper_app	65%	100%

Notes

• Both lookups resolve relative to the package location, so the suite relocates assets._PACKAGE_DIR into a tmp_path tree. Asserting against the real checkout would tie the suite to whether make build-app has run, and fail on a fresh clone.
• Covered: a built app resolving, an unbuilt app raising the actionable make build-app message, a static/ directory without an index.html (a stale or partial build must not be served as if it were complete), the Nuxt checkout resolving two levels up, and an installed wheel with no source checkout.
• packages/interloper-app/pyproject.toml carries its own [tool.ruff], and was the only package in the workspace without the "tests/**" per-file-ignores entry, having had no tests until now. Added the same line the other packages use rather than working around it in the test file.
• The Nuxt frontend under packages/interloper-app/app/ is untouched: it has no test runner today, so adding one is a separate decision.

Checks

uv run ruff check, uv run ty check, uv run pytest (2583 passed) all green from the repo root.

By Digitl

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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