perf: convert_alpha() cached drawImage surfaces for faster blits

[dmccoystephenson]

Summary

• drawImage already caches the loaded and scaled surface by filePath to avoid repeating the expensive load/scale work every frame, but the cached surface itself was never converted to the display's pixel format.
• An unconverted pygame.Surface is reformatted on every single blit() call (pygame's documented perf guidance is to convert()/convert_alpha() once after loading). Doing that conversion when the surface first enters the cache keeps that reformat cost out of the per-frame render path the cache exists to protect.
• convert_alpha() (rather than convert()) also preserves per-pixel alpha for images that have it, so transparent PNGs continue to render correctly.

No public API surface changed — Graphik.drawImage's signature and observable caching behavior (per-path cache, rescale-on-size-change) are unchanged; this is purely an internal perf improvement.

Test plan

• python3 -m py_compile src/main/python/preponderous/graphik/graphik.py
• python3 -m pytest — 25 passed (was 24; added test_draw_image_converts_loaded_surface_for_faster_blits)
• Regression-verified empirically: stashed the production fix, confirmed the new test fails (SRCALPHA flag absent), restored the fix, confirmed it passes.
• Existing drawImage correctness/caching tests (pixel-color, load-count, scale-count, rescale-on-size-change) all still pass unchanged.

No tracking issue — gap found during triage; the backlog (issues/PRs) was empty at the start of this cycle.

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

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — only graphik.py (the one-line cache-site change) and its test file are touched; no unrelated formatting/renames.
• Tests-new: PASS — test_draw_image_converts_loaded_surface_for_faster_blits exercises the new conversion via the surface actually passed to pygame.transform.scale.
• Tests-fix (empirical): PASS — stashed the production change, confirmed the new test fails (SRCALPHA flag absent on the pre-fix surface), popped the stash, confirmed it passes. Not scored from reasoning alone.
• Sibling structure: PASS — new test follows the existing drawImage test group's structure/naming and reuses the existing _count_image_scales monkeypatch helper rather than inventing a new pattern.
• Sibling renames: N/A — no identifier renamed.
• Docs: PASS — checked README's drawImage bullet against the new code; the documented caching contract (per-path cache, rescale on size change) is unchanged, so no doc update was needed.
• Issue resolution: N/A — no tracking issue; gap found during triage (backlog was empty at cycle start), noted in the PR body.
• Manual validation: PASS — py_compile, the import smoke test, and pytest (25 passed, up from 24) all green on the PR head commit (a4113c5).
• camelCase: N/A — no new/renamed public method; drawImage's name and signature are unchanged.
• Backward-compat: PASS — drawImage(filePath, xpos, ypos, width, height) signature and observable behavior (per-path caching, rescale-on-size-change, missing-file error) are unchanged; only the internal pixel format of the cached surface changed. Consumer-impact search (gh search code) is blocked pending approval in this headless dispatch (tracked as graphik-dev-loop#14) — not run, but the gate is scoped to changes that rename/remove/alter an existing public member's signature, which this is not, so it does not apply here.
• Headless: PASS — the new test runs under the suite's existing SDL_VIDEODRIVER=dummy/SDL_AUDIODRIVER=dummy headless setup (conftest.py), no real display/audio required.
• Version sync: N/A — no version string changed.
• No new deps: PASS — convert_alpha() is a pygame.Surface method already available via the existing pygame dependency; no new import.

Summary: internal perf-only fix to drawImage's image cache (adds convert_alpha() once per cached path so per-frame blits don't pay the pixel-format conversion cost every call), covered by an empirically-verified regression test. No public API change.

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drafted by Claude on behalf of Daniel Stephenson