fix(value): guard collection access on atoms

[belowzeroff]

Summary

Guard collection-only accessors before calling vector/list FFI paths:

• Value::get(idx) now returns a binding error when called on a non-vector/non-list value.
• Value::is_null_at(idx) now returns false for non-vectors and out-of-range indexes before calling ray_vec_is_null.

User-visible bug

These are safe Rust APIs, so calling them on an atom should not reach C functions that expect vector/list layout.

Before this change, code like this could treat an atom as if it had collection storage:

let atom = Value::i64(42);
let _ = atom.get(0);
let _ = atom.is_null_at(0);

After this change:

atom.get(0)        -> Err(... value is not a vector or list ...)
atom.is_null_at(0) -> false

This keeps the existing is_null_at signature, avoiding an API break, while making the FFI boundary total for non-vector values.

Tests

• Added collection_access_rejects_atoms regression coverage.
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• env RUST_TEST_THREADS=1 cargo test --workspace