Out-of-bounds read when decoding internal keys shorter than 16 bytes

Summary

`InternalKeyRef::from_encoded` is a safe constructor whose only length check is `debug_assert!(encoded.len() >= 16)`, which is compiled out in release builds. Its accessors perform unchecked length arithmetic on the slice: `user_key()` slices `&encoded[..len - 16]`, `trailer()`/`seq_num()` pass `len - 16` and `timestamp()` passes `len - 8` to `read_u64_be`, which computes `buffer.as_ptr().add(offset)` and performs an 8-byte load. For a short slice the subtraction underflows `usize`, the pointer wraps below the allocation and eight bytes before the buffer are read (undefined behavior; confirmed with AddressSanitizer as a `heap-buffer-overflow`). In debug builds the same call panics instead.

The same invariant is missing at the on-disk decode boundary: `BlockIterator::seek_next_entry` rebuilds the current key from block bytes with upper-bound checks only, and `BlockIterator::key()` wraps it in `InternalKeyRef`, which `surrealdb`'s `kv-surrealkv` backend then reads through `key_ref.timestamp()`. A crafted or corrupted SSTable therefore reaches the same unchecked subtraction without any `unsafe` at the call site.

`1.2.0` rejects internal keys shorter than their 8-byte trailer before any accessor computes an offset and makes the accessors bounds-checked (fix in commit `ac642f9`, `surrealdb` 4.0.0 ships `surrealkv` 1.2.0).