test: pin drawImage's preservation of per-pixel alpha

[dmccoystephenson]

Summary

• A characterization test is added for a behavior that drawImage's implementation comment promises but that no test pinned: convert_alpha() "preserves any per-pixel alpha" the loaded file carries, so a sprite's transparent pixels leave the display underneath visible.
• The existing test_draw_image_converts_loaded_surface_for_faster_blits only asserts that the cached surface gained the SRCALPHA flag. A conversion that adds an alpha channel while discarding the file's own (e.g. convert().convert_alpha()) satisfies that check yet paints transparent sprite backgrounds opaque — a regression every consumer drawing sprites (Roam, Ophidian, etc.) would notice.
• The fixture is a 32-bit BMP, which keeps its alpha channel without SDL_image, matching the portability rationale of the existing _write_solid_image helper.
• No production code is changed. This is a Stage B (unit-test expansion) cycle.

Mutation check

Performed by editing graphik.py temporarily and restoring it afterwards:

drawImage load line	..._converts_loaded_surface...	new test
convert_alpha() (current)	PASS	PASS
convert()	FAIL	FAIL
convert().convert_alpha()	PASS	FAIL

The last row is the gap this test closes.

Test plan

• python3 -m py_compile src/main/python/preponderous/graphik/graphik.py
• Import smoke test for preponderous.graphik.graphik
• pytest (headless via conftest.py): 53 passed (pygame 2.1.2 / SDL 2.0.16, run locally)
• Mutation check above

Deferred issues

All open issues were deferred because each one waits on a maintainer decision:

• release: version tags are pushed without the 'v' prefix publish.yml requires, so no release has ever been published #34 needs a decision on release tagging vs. the publish.yml trigger, and changes to .github/workflows/ are excluded from autonomous work.
• ci: test.yml's header comment claims it runs on every push, but pushes outside main are only covered via pull_request #39 is a comment fix confined to .github/workflows/test.yml, which is also excluded from autonomous work.
• packaging: the empty preponderous/__init__.py ships a regular package, which will collide with any future preponderous.* distribution #40 depends on the convention used by the sibling preponderous.* projects, which cannot be determined from inside this repository.

No tracking issue. The gap was found during a test-coverage sweep in triage.

This PR description was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

🤖 Generated with Claude Code

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drafted by Claude on behalf of Daniel Stephenson

[dmccoystephenson]

Self-review rubric (performed inline; not an independent review):

• Scope: PASS — the diff touches only src/test/python/preponderous/graphik/test_graphik.py (+26 lines); no production code, docs, or version files are changed.
• Tests-new: PASS (n/a) — no public method is added.
• Tests-fix: PASS (n/a, no fix) — as a mutation check instead, graphik.py's load line was temporarily edited: convert().convert_alpha() fails only the new test (the existing SRCALPHA-flag test still passes), and convert() fails both. The source was restored and git status confirmed it unchanged.
• Sibling structure: PASS — the test sits beside the other drawImage tests, reuses _make_graphik/_rgb, uses camelCase locals like its neighbours, and uses a BMP fixture as _write_solid_image does.
• Sibling renames: PASS (n/a) — nothing is renamed.
• Docs: PASS — the behavior pinned is already stated in drawImage's implementation comment; README, RELEASING.md and the docstrings need no change.
• Issue resolution: PASS (n/a) — no Closes reference; a gap found during triage.
• Manual validation: PASS — py_compile and the import smoke test succeed locally, and pytest ran 53 tests with 53 passing. The CI Test matrix (3.9–3.13) is green on the PR head.
• camelCase / Backward-compat / Version sync / No new deps: PASS — no public surface, versions, or imports are changed.
• Headless: PASS — the test runs under the conftest.py dummy SDL drivers, locally and in CI.

Judgment call: the BMP fixture relies on SDL keeping a 32-bit BMP's alpha channel on load. Both SDL 2.0.16 locally and the CI wheels do. A PNG would be the more realistic sprite format but needs SDL_image, so BMP was kept for consistency with the existing fixture's portability rationale.

Requesting Copilot as a reviewer failed (Could not resolve user with login 'copilot'), so this self-review serves as the review step.

This comment was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

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drafted by Claude on behalf of Daniel Stephenson