guard guest cursor and delta in wasi-keyvalue in-memory store

[netliomax25-code]

1. list_keys casts the guest-supplied cursor with as usize and slices &keys[cursor..], so a cursor past the number of keys panics the host with an out-of-range slice, and the cast also truncates the u64 on 32-bit hosts.
2. increment computes current_value + delta, which panics on overflow in debug builds and silently wraps to a smaller value in release, so the returned counter can go backwards.

Clamped the cursor to keys.len() (via try_from + min) and switched increment to checked_add, returning an error on overflow. Both values come straight from the guest, so the checks live in the callee. Added unit tests that panic on the current code and pass with the fix.