test: add fuzz testing + replace get_* with try_get_* to avoid panics

[matthias-wright]

This adds fuzz testing via cargo-fuzz for three categories:

• Parser targets: every Read impl in summit-types. Arbitrary bytes must parse to Ok / Err without panicking, and successful decodes roundtrip to byte-identical output.
• Non-codec targets: ssz_tree_key::parse_key, derive_child_public, SszProof::verify with adversarial inputs.
• Property-based targets: WithdrawalQueue op-sequence invariants, ExtPrivateKey sign-verify roundtrip, SSZ incremental-update-vs-rebuild root equality, and generate→verify proof roundtrip across all proof kinds.

Also replaces calls to get_u64 (and similar) with try_get_u64 to avoid panics if there aren't enough bytes to fill the type.