gallery: validate declared installable deps; add pygraphics to apollo deps; refresh requirements and tests

[bdbarnett]

Motivation

• Ensure gallery examples declare any installable top-level PyDevices deps they actually import so missing runtime packages are detected early.
• Keep requirements.txt and refresh-requirements.py in sync with packages that publish from the audio components repo.
• Fix the Apollo example metadata to list pygraphics as a dependency.

Description

• Add INSTALLABLE_DEPS and an AST-based imported_top_level_modules helper plus validate_example_deps to detect missing or unused declared installable deps, and call it from main in scripts/gallery_generator.py.
• Import ast where needed and wire dependency validation into the discovery flow.
• Update lib/examples/apollo/apollo.py header comment to include pygraphics in the # deps: line.
• Adjust requirements.txt ordering to include pydevices-audioinstruments and pydevices-audioeffects in the installation order.
• Update scripts/refresh-requirements.py PACKAGE_ORDER with comments and the two audio component entries so the refresh script keeps them.
• Add a unit test in tests/test_gallery_screenshots.py that exercises validate_example_deps for missing/unused-dep detection.
• Refactor tests/test_peterhinch_page.py into a unittest.TestCase class and make it runnable as a script with a if __name__ == "__main__": unittest.main() entry.

Testing

• Ran the updated gallery unit tests via python -m unittest tests.test_gallery_screenshots and the new dependency checks passed.
• Ran python -m unittest tests.test_peterhinch_page and the refactored page tests passed.
• Ran the modified tests as part of the local unit test run (python -m unittest) and observed no failures for the changed files.

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Codex Task

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Verification by the reviewer (2026-09-06)

The branch was fetched and run on the maintainer's machine beside the pydevices sibling checkout, which tests/test_browser_url.py needs.

Command	Result
python -m unittest tests.test_peterhinch_page	Ran 17 tests … OK (0 collected before this PR)
python -m unittest discover -s tests	Ran 60 tests … OK (43 before)
scripts/gallery_generator.py --check	exit 0; 55 gallery demo(s) (47 module, 8 manifest; 5 featured; 2 in a new window); 17 local-only; three reverse-check warnings (tiny_toasters declares palettes, touch_gui_simpletest declares pygraphics, widgets_locker_kiosk declares audioif; none imported)
Planted fault: pygraphics removed from lib/examples/apollo/apollo.py's # deps: header, then --check	exit 1: gallery dependency errors: lib/examples/apollo/apollo.py: missing # deps: pygraphics — header restored afterwards

The apollo header change is a real undeclared dependency the new guard found: on main the file declares # deps: palettes and imports pygraphics, so the Apollo DSKY demo has been failing on the deployed gallery the same way drum_machine did before #120 was filed.

The requirements.txt / refresh-requirements.py lines in this diff are identical to what main already carries in 8268d3aa; the branch re-applied them on the older base, and the merge resolves them as one change.