Per-frame first-visible render anchor kills the screen shake

The frame composer COMPACTED visible scanlines (stacked top-down, skipping blanked lines): any mid-frame line drop made everything below it breathe upward, and frame-start variance translated content. v2 policy: each frame's first visible scanline pins to row 0 and all other lines map by ABSOLUTE offset from it — no interior compaction, no translation; gaps render black; bottom score/lives bands stay locked.

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