Share link length and functionality for Breakscale designs.

A share link for a nine node design is about 3,900 characters today. Discord's message limit is 2,000, so pasting one there does not produce a link at all: it silently becomes a `message.txt` attachment, and whoever receives it has to download a text file and paste the contents by hand. That is a broken share rather than an ugly one.

## Why the link is that long

A design is carried entirely in the URL fragment (`src/share.ts`), deflated and base64url encoded, so nothing is ever uploaded and a link keeps working whatever happens to this project. The length is the price of that, and it scales with the design.

Measured across the bundled examples: median 1,034 characters, longest 2,745. A hand built design with more nodes goes past 3,900.

## Roughly half of it is redundant

Measured on `full-stack`, nine nodes:

```
total JSON 4379 bytes
config fields 2356 bytes
of which equal to defaultConfig(kind) 2070 bytes (47% of the payload)
coordinates 153 bytes
```

Every node carries its whole config even when nothing was changed, and the decoder could rebuild all of it from `defaultConfig(kind)`. Stripping defaults, shortening field names and rounding coordinates should roughly halve the JSON before deflate. That is very likely enough to clear 2,000 characters for ordinary designs, with no server and no format concept beyond what `share.ts` already has.

The version prefix exists for exactly this: `d1.` is read by the current decoder, so a `d2.` can drop def...