[codec] Add Checksummed type

[crazywriter1]

Closes #1704.

Summary

Adds Checksummed<T, C>, a wrapper that appends a checksum of a value's encoding when writing and verifies it when reading. This lets any Codec type detect accidental corruption at rest or in transit without each caller hand-rolling the framing.

The wrapper is generic over a new Checksummer trait rather than over a hasher from commonware-cryptography, since the hashing crates depend on commonware-codec and the reverse dependency isn't possible. Checksummer::Digest is any FixedSize codec type, so implementations can range from a CRC to a cryptographic digest.

Details worth calling out for review:

• Read::Cfg forwards to T::Cfg, so wrapping a type doesn't change how it is configured. Config-driven failures (e.g. a RangeCfg rejection) surface before any checksum work.
• Verification re-encodes the decoded value instead of retaining the consumed bytes, which avoids buffering but assumes canonical encodings. Every implementation in this crate is canonical; for anything else, a non-canonical encoding that decodes to the same value is rejected as a mismatch. This is documented on the type.
• The PhantomData<fn() -> C> marker keeps the wrapper's auto traits independent of C.
• Docs note explicitly that a checksum is not authentication.

Testing

• Unit tests cover round trips for fixed- and variable-size inner types, config forwarding, encode_size agreement with the written bytes, corrupted payload and corrupted checksum rejection, a missing trailing checksum, and that read consumes exactly one value from a buffer holding more.
• Doctests on both Checksummer and Checksummed.
• just pre-pr passes, including the no_std build (the type adds no std usage).

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