Add hex sticker

## What

Add a hex sticker for greta.gam.

The sticker is designed and rendered. It comes from `greta.hexes`, which holds
the whole family — that repo is not pushed yet, so the file is currently local:

```
png/light/stacked/greta-gam.png
```

600 dpi, 43.9 × 50.8 mm, transparent outside the hexagon.

## The symbol

A **penalised smooth through a cloud of scattered points** — passing *through*
them rather than joining them, which is what a smooth does, with the scatter wide
enough to see the difference.

It carried knot ticks along the bottom at one point. They cost the lower sixth of
the space and at sticker size read as damage rather than notation, so dropping
them made the curve bigger *and* the symbol cleaner.

## The family

All four extension stickers share:

- the **greta wordmark at the original hex's size and height** — measured on the
rendered PNGs, the ink spans 0.362–0.622 of the image height against
`greta_hex.png`'s 0.362–0.623
- **Mulish** — Google renamed Muli to Mulish, so `font_add_google("Muli")` no
longer resolves, but it is the same face the original used
- the `light` theme — `#E0D2EE` fill, `#6B4A8B` type
- a hairline border, so the symbol is what distinguishes a sticker rather than
the frame
- a 2mm safe margin, verified against the rendered pixels rather than assumed

**greta's own hex is unchanged.** It stays the original at
[`logos/greta_hex.png`](https://github.com/greta-dev/greta/blob/main/logos/greta_hex.png);
these are designed to sit alongside it, not to replace it.

## To do

- [ ] copy the PNG into the package
- [ ] `usethis::use_logo("greta-gam.png")`
- [ ] check the logo renders in the README
- [ ] confirm pkgdown picks it up

Start with the local asset at png/light/stacked/greta-gam.png and the package's README, then use usethis::use_logo("greta-gam.png"). Check that the logo renders in the README and confirm pkgdown picks it up; the greta hex at logos/greta_hex.png should remain unchanged.

Written by the indexing model from the issue text.