fix(compaction): persist keys in lookup SST files while first-row merge function

[lszskye]

Purpose

Linked issue: close #251

First-row lookup uses LookupLevels<bool> because it only needs to determine whether a primary key already exists.

The previous implementation special-cased bool by creating and immediately closing the lookup writer. This produced a valid but empty SST file. As a result, a compaction could incorrectly retain a newer row even though the first row for that key already existed.

This change:

• removes the bool-specific empty SST shortcut;
• writes serialized primary keys with empty value payloads;
• uses a key-only read schema for first-row lookup, avoiding unnecessary value-column reads.

Tests

Added LookupMergeTreeCompactRewriterTest.TestFirstRowLooksUpExistingKeys, covering:

• an existing key=1 in L2;
• key=1 and key=2 in L0;
• compaction of only L0 into L1;
• verification that the output contains only key=2.

API and Format

No public API or protocol changes.

Documentation

No documentation changes are required.

Generative AI tooling

Generated-by: Codex (GPT-5)

[lxy-9602]

+1