Rewrite PPU pipeline for DMG-01 accuracy

[m0xsec]

Summary

• replace the PPU with a cycle-driven DMG-01 implementation that manages LCD enable, modes, and STAT interrupts
• implement full background/window/sprite rendering with palette mapping and correct tile addressing
• enforce VRAM/OAM access timing rules and expose the screen pixel count constant used by the UI

Testing

• cargo test -q

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Handle WX<7 without wrapping window start

The window X origin is computed with self.wx.wrapping_sub(7) and later compared to x as u8, so any WX value below 7 wraps to 0xF9–0xFF and the using_window check never fires. On real hardware WX−7 can be negative, allowing the window to appear from the left edge when WX is 0–6 (common for HUDs). With the current unsigned wrap the window is completely disabled for those values, so games that place the window at the left edge will render nothing.

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[chatgpt-codex-connector]

Preserve negative sprite X positions

Sprite X positions are calculated as data[base + 1] as i16 - 8 but are immediately cast to u8 when pushed into line_sprites. For sprites whose X is less than 8 (partially off the left edge), this cast wraps negative coordinates to 0xF8+, and the later comparison (x as i16) < (*sprite_x as i16) causes the sprite to be skipped for every screen X. As a result, sprites near the left border never render even though the hardware draws their visible portion.

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