Bounds-check per-entry key read in compact_tuple_sketch::deserialize

[SavicStefan]

The entry-reading loop reads each entry as an 8-byte key followed by a Summary-width summary. The summary read is passed the remaining capacity and bounds-checks itself, but the key read is an unchecked fixed-size copy_from_mem that relies solely on the pre-loop keys-only reservation, which assumes every entry's summary occupies the width Summary serializes to.

When that assumption does not hold (a truncated buffer, or a buffer whose serialized summary width differs from the Summary it is deserialized as) the read cursor advances past the data. num_entries is read from the preamble and is unaffected, so the loop runs the full count and a later key read walks off the end of the buffer: silent on a normal build, a heap-buffer-overflow under AddressSanitizer.

Add an ensure_minimum_memory check before the key read, mirroring the remaining-capacity check the summary read already performs and the up-front size validation the compact theta parser does. A malformed buffer now throws std::out_of_range instead of reading out of bounds.

Adds a tuple_sketch_test case that deserializes a float-summary sketch as a double-summary sketch and asserts std::out_of_range.

[proost]

The implementation change looks appropriate to me. But I suggest revising the test case to focus on truncated/malformed-buffer safety.

[proost]

Thank you!