Handle CustomTypeSlim parameter type (v18)

## Context
Not implemented in `photon-parser` v18 — not yet reached by Albion Online traffic.

## Type
| Name | Description |
|---|---|
| `CustomTypeSlim` | Slim custom object |

## Tasks
- [ ] Identify wire format for `CustomTypeSlim`
- [ ] Implement decoding
- [ ] Add test case

## Labels
`photon-parser` `v18` `parameter-type` `todo`