Fix integer multiplication overflow in jellyfish lake density rendering

[Copilot]

3*width*height in jellyfish.cc performs all arithmetic in unsigned int, risking overflow before the result is widened to size_t for the vector constructor — a problem on large canvases where width*height exceeds 2³²/3 ≈ 1.4 billion.

Change

Cast the leading literal to size_t so the entire multiplication chain runs in 64-bit arithmetic:

// before
vector<unsigned char> pp(3*width*height, 255);

// after
vector<unsigned char> pp(size_t(3)*width*height, 255);

Addresses CodeQL alert cpp/integer-multiplication-cast-to-long (#384).

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This change is 

[Copilot]

Pull request overview

This PR fixes a potential integer-overflow bug in lake_t::draw_density() when allocating the pixel buffer for density rendering, by widening the multiplication used to compute the vector size so it can’t overflow in 32-bit unsigned arithmetic on large canvases.

Changes:

• Widen 3*width*height to size_t arithmetic by casting the leading literal to size_t in the pp buffer allocation.

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