Add hex sticker

## What

Add a hex sticker for greta.dynamics.

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-dynamics.png
```

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

## The symbol

A **Lotka–Volterra predator–prey cycle**, integrated with RK4 — the same kind of
thing `ode_solve()` is for. Prey solid, predator dashed.

The detail to look for is that the predator's peak **lags** the prey's. That falls
out of the integration; it is not something you would get by offsetting two sine
waves. It replaced a spiral converging to a fixed point, which said "some
dynamical system" rather than one anybody recognises.

## 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-dynamics.png")`
- [ ] check the logo renders in the README
- [ ] confirm pkgdown picks it up

Start by checking the package's existing logo assets and README, then obtain the rendered file at png/light/stacked/greta-dynamics.png from the issue author or source repository. Copy it into the package, run usethis::use_logo("greta-dynamics.png"), and verify that the logo appears in the README and is picked up by pkgdown.

Written by the indexing model from the issue text.