Handle partial-word serde byte input - #3755

Summary

Fixes #3754.

risc0_zkvm::serde::from_slice can currently panic when called with byte input whose length is not a complete number of u32 words, e.g. from_slice::<u32, u8>(&[0, 1, 2]).

This PR adds a small pre-check that returns Error::DeserializeUnexpectedEnd for partial-word input before attempting the bytemuck cast. It preserves the existing unaligned-but-complete byte-slice path.

Implementation note

This also changes the remaining bytemuck catch-all arm from a panic to Err(Error::NotSupported). The partial-word length precheck is the actual fix for this issue; the catch-all change keeps from_slice aligned with its Result API. If maintainers prefer the smallest possible behavior change, I can drop that part and leave the catch-all arm unchanged.

Tests

cargo fmt --check
git diff --check
RISC0_SKIP_BUILD=1 cargo test -p risc0-zkvm --lib serde::deserializer::tests -- --nocapture

RISC0_SKIP_BUILD=1 was needed in my Termux/Android environment because guest build scripts otherwise fail with UnsupportedPlatform("unknown OS: android").