serdect version skew

`cx448` depends on:

```
crypto-bigint = { version = "0.5.5", features = ["generic-array"], default-features = false }
serdect = { version = "0.3.0", optional = true }
```

but `crypto-bigint` 0.5.5 [depends on an earlier version of `serdect`](https://github.com/RustCrypto/crypto-bigint/blob/395bb171178990a93ef571664271dabc50749043/Cargo.toml#L27):

```
serdect = { version = "0.2", optional = true, default-features = false }
```

This is troubling, because it requires two semver-incompatible versions of `serdect`, in particular across [a major change to the wire format](https://github.com/RustCrypto/formats/blob/master/serdect/CHANGELOG.md#030-2025-01-06):

```
## 0.3.0 (2025-01-06)

NOTE: this release includes major breaking changes to the wire format, namely
all bytestrings now include a length prefix, even when serializing fixed-size
arrays. This is intended to work around deficiencies in the `serde` API
(see serde-rs/serde#2120) as well as serde-based format implementations which
have variable-time behavior when using `serialize_tuple`.

Any binary data serialized with previous versions of `serdect` now needs a
length prefix prepended to the data, which will vary depending on the
particular data format.

### Changed
- Switch to length-prefixed encoding using the `serialize_bytes` method ([#1112], [#1515])
- MSRV 1.70 ([#1244])

[#1112]: https://github.com/RustCrypto/formats/pull/1112
[#1515]: https://github.com/RustCrypto/formats/pull/1515
[#1244]: https://github.com/RustCrypto/formats/pull/1244

```

The implication is that to build `cx448` i need to have two different versions of `serdect` available, each of which produces (and consumes) a different wireformat. Is that deliberate?

Start by inspecting cx448's dependency declarations and Cargo's resolved dependency graph, then compare the referenced crypto-bigint Cargo.toml entry with serdect 0.3.0 and its changelog. Determine whether the two serdect versions can coexist safely across the affected wire format; done should include a decided dependency strategy and evidence that the resulting build and serialization behavior are correct.

Written by the indexing model from the issue text.