runtime: reset PPU scroll after VRAM buffer drain

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Each $2006 write inside __vram_buf_drain updates the PPU's `t`
(scroll) register, so leaving it pointing at the last buffer
entry's address shifted the next frame's rendering up/right by
however many cells we wrote past $2000. Reset by writing $00 to
$2006 twice (clears `t` and resets the write-toggle to high)
followed by $00 to $2005 twice (zero X/Y scroll). The HUD demo
golden flips from "smileys offset by ~16px" to the intended
red bar with white hearts and a yellow score digit.

https://claude.ai/code/session_01F7dHsgh7UX7SAK3wZ7JiKc

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Claude 2026-04-18 22:57:46 +00:00

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commit 1c00d20121

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examples/hud_demo.nes

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examples/vram_buffer_demo.nes

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