Format RGB channels as hex

Colour pickers and interpolation code work in numbers, but CSS and <input type="color"> want #rrggbb. This conversion is where off-by-one padding bugs live: channel 1 must render as 01, not 1.

Implement rgbToHex(r, g, b). Each channel is first rounded with Math.round, then clamped into the range 0 to 255, then rendered as exactly two lowercase hex digits, zero-padded on the left.

Return the three channels concatenated after a single #, so the result is always exactly seven characters, lowercase — for example "#ff8800".

What it has to do

• Return a seven character string starting with #, using lowercase hex digits.
• Zero-pad each channel to two digits.
• Round fractional channel values with Math.round.
• Clamp values below 0 to 0 and above 255 to 255.

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