fix(milkdrop): a still motion-vector stub points up, as in MilkDrop

[CaYatur]

When a motion vector has no length at all (mv_l = 0, or nothing moves), MilkDrop draws a one-texel stub with dx = dy = min_len. MilkDrop's fy runs bottom-up, so the stub goes right and up; ours runs top-down and kept the sign, so it went right and down. Vectors with a real length were already correct.

• src/visualizer/modes/milkdrop.js: ddy = -minLen in the zero-length branch.
• tests/milkdrop-motion-vectors.test.js: a still field (mv_l = 0) draws stubs one texel right and one texel up. Fails without the fix (yukarı: 0.00104 / -0.00104).
• CHANGELOG "Unreleased" entry; test count 3004 → 3005.

Found while checking our motion-vector code against MilkDrop's for #750.

Refs #560, #695

[cayadraft]

@CaYatur

CaYaDraft — pull request

Summary
This pull request fixes a rendering bug in the MilkDrop mode where still motion vectors (zero length) incorrectly drew stubs downward instead of upward. The issue arose because the code kept the original sign for vertical movement while reversing the drawing order, causing the visual artifact to point down rather than up like the reference application.

Files

• src/visualizer/modes/milkdrop.js: Corrected the vertical offset for zero-length motion vectors to draw upward.
• tests/milkdrop-motion-vectors.test.js: Added a test case verifying that still fields render stubs one texel right and one texel up.
• CHANGELOG.md, README.md, README.tr.md, ROADMAP.md: Updated documentation and changelog entries to reflect the fix and increased test count.

Note
Nothing blocking is visible from the text.

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