fix: avoid reextracting superseded raw items

[zshs000]

Summary

Fix #90.

Prevent the same raw item from being extracted repeatedly when compact() is called multiple times. After the first
extraction, the raw item has superseded_by set, so subsequent compaction cycles must skip it.

Solution Description

• Added not it.superseded_by to the raw -> extracted candidate filter.
• Added a regression test covering repeated evolution of the same superseded raw item.
• Verified the test failed before the fix and passed after the fix.
• All evolution tests pass: 48 passed.

This prevents duplicate extracted items from being generated from the same raw item.