fix(hll): update silently dropped after deserializing a compact List sketch (#117)

When an `HllSketch` in **List mode** is serialized it uses compact format:
only the live coupons are written to disk, with no trailing `COUPON_EMPTY`
sentinels. On deserialization, the container was allocated with exactly
`coupon_count` slots, leaving the array fully packed.

`list::update()` scans linearly for a `COUPON_EMPTY` (`0`) sentinel to find
an insertion slot. Finding none, it fell through silently — discarding every
value added after the round-trip.

Always allocate `1 << lg_arr` slots (initialized to `COUPON_EMPTY`) in
`List::deserialize()`, reading only `coupon_count` elements from the byte
stream in compact mode. The trailing empty slots are then available as
sentinels for subsequent `update()` calls.

Fixes #115.