fix: free large-value segment pages on full-partition Truncate

[thweetkomputer]

Problem

Truncate's full-partition fast path (empty position) iterated only the data mapper and FreePage'd its pages — it never touched the segment mapper. For a partition holding large values the segment mapping stayed fully populated, so the empty snapshot flushed by UpdateMeta serialized it verbatim:

• GC's manifest replay kept every segment file alive forever (only DropTable would reclaim them);
• segment compaction counted them as live, so amplification never triggered;
• the stale segment mapping was copied into every later archive/branch snapshot.

Reads correctly saw an empty partition, so this is a silent space leak (disk and cloud) + snapshot pollution, not a correctness bug. The partial-truncate path already frees these via DelLargeValue.

Fix

Free the segment pages too, mirroring the data-mapper loop, so the flushed segment mapping is empty and GC reclaims the files. The empty-snapshot flush serializes the current segment mapping (not deltas), so no delta recording is needed.

Test

large_value_gc: full-truncating a partition of pinned large values leaves no segment files after GC. Verified it fails without the fix (the files are retained).

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between e8cd94e and b44e74f.

📒 Files selected for processing (2)

• src/tasks/batch_write_task.cpp
• tests/large_value_gc.cpp

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/truncate-segment-leak

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