Paint the scene only on the theme's own page background

[NguyenDTAnh]

Fixes the flicker in #37.

On a dark theme the wallpaper paints into dialogs, menus, the prompt, and the message, tool and code blocks in a session. The engine guessed the page color as the most common one in the frame, and on a session with a screenful of messages or tool output the most common color is those blocks' own surface - so the engine took it for the page and painted the scene into them. It recomputes its surfaces, and the tint it applies to them, every tenth frame and holds color changes under 3 levels back, so those cells flip back and forth, which reads as the block flashing.

The plugin already has the real page color: context.theme.background.base. This passes it into the engine and paints the scene only on cells the UI filled with it. Every other background - a dialog, a menu, a message, a tool block, the prompt - belongs to the UI. When the page color is not passed in the engine still falls back to the most common color, so dev/mock.ts and the checks are unchanged.

Measured on a real session (OpenCode 2.0.28, 160x45, dusk-aurora, ocean):

	before	after
cells the UI painted with a non-page background that the engine repainted	783	0
engine octant glyphs on a non-page background	252	0

dev/panels.ts covers the dialog case on a mock screen: a dialog window over a dusk-aurora-like scene carries no scene glyphs and is not repainted on surface recounts, on every wallpaper it is run against.

• bun dev/panels.ts ocean (and the other wallpapers): ok
• bun dev/check.ts --quiet: 126 runs, 0 failed
• bun run typecheck: clean for src/ and tui.tsx

The scene no longer glows through surfaces that are a touch off the page color, which is what TINT_RANGE did for animated highlights; a highlight that is not the page color now reads as the UI's own.

[NguyenDTAnh]

Closing: the approach is wrong. The engine reads each cell's background color to decide what the scene may paint, but OpenTUI renders only the cells that changed, so a cell the UI did not repaint still holds the scene color from a previous frame. Restricting the scene to the theme's page color therefore makes the wallpaper vanish, and the original heuristic paints into dialogs, menus and message blocks. The right fix needs the frame diff (current buffer vs the buffer the engine wrote last frame) or a per-frame ownership mask from the host; I've rewritten issue #37 with the evidence.