Adjustments to number glyphs

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I've concentrated on the number glyphs in this PR to optimise colour, curves and optical balance. I also adjusted the shapes slightly for the light versions, to smooth out curves and also improve interpolation with the new black shapes.

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I also just fixed a bit of uneven thickness in the upper part of /3/ that was catching my eye :)

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I think it probably depends on personal preferences, I had a look at the two reference fonts Montserrat and Karla to see how they handle this.

Montserrat does duplicate the /6/ and rotate it around to make a /9/, they have the component in the background layer and have redrawn the glyph from it:

However Karla have actually modified the curves of the /9/ slightly for optical balance - it may be also a stylistic choice. Here I've put the flipped /6/ is in the background layer to demonstrate:

It appears in Karla that the tail of the /9/ is modified a little to bring it in slightly more and close off the shape. And the bowl of the /9/ is also a bit higher than it would be in the /6/.

I'll let you make the final call about the approach :)

Also are you happy with the PRs being this size or do you prefer smaller PR's to seperate changes to each glyph?

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I also had a look at Poppins, out of interest since it's a design that's close to Blueberry. They have just a slight difference between /6 and /9 - seems like the /9 is a little thicker in the top right and slightly less in the bottom left. That might be a good visual adjustment to make.