feat(core): Chunk.aliases 필드 (doc-side expansion)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/kebab-store-sqlite/tests/idempotency.rs

@@ -98,6 +98,7 @@ fn make_chunks(doc_id: &DocumentId) -> Vec<Chunk> {
         chunker_version: ChunkerVersion("md-heading-v1".into()),
         policy_hash: "deadbeefdeadbeef".into(),
         tokenized_korean_text: None,
+        aliases: None,
     }]
 }